The only issue with that idea is then I have the expense of an electrice drivetrain and a mechanical one.
The current state of the project is motorless so it will need a new motor and transmission as of its current state otherwise keeping the factory motor intact could have been an option.
I would prefer to go with an all electric drivetrain so in the future I can utilize upgraded battery technology to gain higher ranges and eventually downsize the generator needed... and also I wont have the double expense of an electric drivetrain and a mechanical one.
Yes, that's true of any hybrid; you need something to handle the engine output. In a series hybrid you have the same transmission as a straight EV (with one motor), but you need a generator on the engine capable of taking the entire engine output, which is not cheap or small.
Even with no engine (and no transmission?) to start with, an engine and transmission (presumably not the same as the Volvo originals) would be less expensive than an electric powertrain... but I realize that's not the purpose of the project.
from what ive read and calculated I will need at least 1kwh / mile to move this thing fully loaded...
That's roughly what I'm thinking, on the basis that both rolling resistance and aero drag will be about three times as great as for a large modern car.
What would I need for a generator wise to produce enough electricity to keep be moving forward... This particular RV wouldnt be used to cruise long highways at 70mph it would be back roads with frequent stops at 35-55mph most likely on the dirt.
If you take either the series hybrid approach, or a parallel system with enough electric power to make up the difference between a small engine and whatever is needed at maximum power demand, then the engine can be sized for not much more than average power demand. There are very few production series hybrid light vehicles; the BMW i3 REX does have only a small engine, while the Honda Accord has nearly the same engine power as the non-hybrid. The BMW i3 REX shows that a small and hard-working scooter engine is not an efficient way to power a car.
As an example, if you're using 1 kWh per mile to run at 60 mph, you're using 60 kW average. That's not much less than the original engine, and if you use any less engine you won't be able to sustain that speed (or a lower speed while climbing), but you're saying that's okay. Perhaps an engine that can produce 30 kW efficiently (rather than at peak output) would keep up suitably.
I think I might pick a Prius engine - the 2ZR-FXE produces anything from about 13 kW to 30 kW while staying within 5% of its impressive peak efficiency (of 220 grams of fuel per kWh of output) - but it's gasoline, not diesel. You might get away with an off-highway equipment (e.g. tractor) diesel of some sort; among road vehicles available in North America the engine from a VW car or the Chevrolet Cruze are the only reasonably small diesels that comes to mind, although of course there are smaller engines from other markets.
In a series hybrid the engine needs a generator; if they are directly linked (not through a gear, belt, or chain drive) then the generator turns at engine speeds and needs to handle the maximum engine output torque.
I would love to make this work as a hybrid but if not my next option is a small mercedes diesel or maybe the new cummins 2.8 repower crate motor.
This is a great example of what the Cummins 2.8 crate motor package is intended for. I don't know what it costs, or if a used Duramax 2.8 or Powerstroke 3.2 might be cheaper (and presumably cheaper to maintain than anything from Mercedes or BMW).
