If the fuel tank is at the centre of gravity, then the engine can't be behind the fuel tank (the trailer would have negative tongue weight without cargo), so it must be ahead of the fuel tank. That's pretty front-heavy. A more practical configuration would put the fuel at the axle, and the engine and generator immediately in front of that.
Of course the fuel could be on top or under the genset, but that's not reasonable for a large tank. It could also go in saddle tanks on each side, I suppose.
Even then, the cargo is all going in front and rear trunks on the trailer. That can work, but doesn't sound like a convenient thing to use, and requires attention to distribution between front and rear. On the other hand, if this is a simple build, the front and rear cargo compartments can just be off-the-shelf cargo boxes.
The axle has two wheels. Side-to-side balance is not incredibly critical on a trailer but important enough you don't want it too far off. Mount the engine such that with no cargo and no fuel the trailer is correctly balanced. Tanks are either to either side of the engine or higher, is usually how mobile generators work. Mount them so that empty or full, the trailer is balanced properly. Hitch weight should be 10% of total loaded trailer weight. Actually usually the tank goes above the engine, but that would put the gas tank in the rear window of the van; commercial mobile generators for commercial construction tend to be pretty high.
Luggage goes where it fits. The van currently has a 25 gallon tank. I can't imagine needing more than 50 gallons, but probably a pair of tanks with a combined capacity of 40 would be most practical. Once the generator (and I'm thinking of buying a commercial built-to-spec motor/generator already on a trailer for this, whether new or used) is mounted and the tanks are mounted, you can look at your trailer to determine the best place for luggage.
No matter what the trailer, balancing and securing the load is critical. This goes for 18-wheelers as well as the one you put behind your lawn mower. So loading the trailer will ALWAYS require more attention than loading your car, and loading your car when it's going to pull a loaded trailer will ALWAYS require more attention than if you just had the car. It's not complicated, it's just important to pay attention. Good tire pressure, good grease in the bearings, good brakes, proper hitch weight for the trailer weight. Proper load balance front-to-back on the tow vehicle. Good lights. There's a check list just like on an airplane.
The problem is that a mechanical injection pump is more complex than a complete gasoline injection system plus a complete ignition system. Modern diesels use electronic injection systems which are similar to gasoline injection systems but at much higher pressure; modern ignition systems have no moving parts and routinely go for the life of the vehicle without failure or repair... and a couple hundred thousand kilometres without even changing the plugs.
On top of that, modern diesels have both diesel particulate filter (DPF) and selective catalytic reduction (SCR) emissions control systems which are far more complex and less reliable than the catalytic converter on a gasoline engine. The problems of making a diesel to meet current emissions regulations are so extreme that Caterpillar dropped out of the business of manufacturing these engines for heavy trucks in response to the regulation changes - they just couldn't make it work economically and reliably. And of course most people have heard of Volkswagen's "dieselgate" nightmare. If you use an old engine you avoid all of that, but it's strange to promote EV conversion (which is often chosen for environmental reasons) then run a filthy engine. If you do, a pre-2007 common-rail electronic injection engine from a light commercial application (not something intended only for cars) is probably the most efficient and reliable choice... although I would want to avoid the parts prices of Mercedes engines.
A good diesel run as intended is more fuel-efficient than a gasoline engine under similar conditions. I was just pointing out that the engines which are available in light vehicles which are optimized for hybrid operation (nearly constant power at an engine speed unrelated to road speed) are those gasoline engines.
Modern gas engines in cars light trucks are more reliable than diesel engines in the same vehicles, due to the problems with current diesel emission controls. Of course heavy diesels are reliable (other than the emission controls), and there is no gasoline comparison (although they would be just as reliable) because diesels are cheaper to fuel - due to both fuel consumption and fuel pricing - so no one builds large commercial gasoline engines.
OK what you're saying is at least plausible. I'll look into that, the emissions specs did ping on my brain a few years back, and I remember the dieselgate thing. As I said I had planned to use a diesel generator set, not mate a generator I found to a motor I found somewhere else. Don't particularly want a stinky engine around though either, so it may be that a gasoline or e85 engine may be a more viable alternative. The hovercraft I built awhile back had a VW inline 4 in it, and I kept the computer and the catalytic on it.
If the van has suitable battery capacity for local use as a pure battery EV (running on battery power only), that capacity will be lots for series hybrid operation with the trailer. A plug-in hybrid car typically has about 16 kWh of capacity; the much bigger van with trailer would be a reasonable plug-in hybrid with about three times that capacity, and you'll want that for routine operation without the trailer anyway. Unless you want long battery-only range with the trailer, I don't see any need to make things complicated (and expensive) by putting more battery in the trailer.
The trailer could be treated as a mobile charging station for the van, but the van will need to accept a charge while driving, which is not normal for EVs. You do need the connection, but there are off-the-shelf connectors for high-rate DC charging of EVs at up to 400 volts and a couple hundred amps.
The space under the floor is at least 8 or 9 inches top to bottom and there's a ton of room between the frame members, especially if you can remove the drive shaft. If rear-mounting the motor(s) pans out, then there is easily enough room between the frame rails to put 5x the battery pack of a plugin hybrid in there. Not saying I'll need it, just saying there's room. There would also be all that room under the hood and in the doghouse.
I was thinking of putting raw 3-phase AC through the trailer connector. Thinking now on it that probably won't work so well, a generator set will probably be either 240 or 480, or maybe 400. There's probably no reason to route around that circuitry, and the EV charging circuitry will be set up for one of those too most likely.
What sort of voltage does a highway-capable (65mph) EV run these days? It would be good to avoid unnecessary voltage conversions through all this, which is why I was thinking of 3-phase through the trailer connector.
I was also thinking of gadget power in the vehicle, we have a ton of phone chargers and tablets and whatever crap we need for baby food. How many people run an AC wall outlet in the car? It's either that or a whole crapload of 12v or USB plugs. Or maybe all of the above.
Back in the day, EV builders had a high voltage battery pack and then a 12v battery for the automotive systems. Is this still the practice or do you run an inverter-pwm regulator for that sort of thing?