You're a little bit all over the map, so, probably best to slow down and make some plans or decisions.
The motor isn't going to be too big of a deal, you can manage that with minimal expense. Ditto for speed controller/inverter.
But batteries is where you're going to have to make some smart decisions.
Batteries are:
- Expensive - Kills a cost-saving project right then and there.
- Heavy - Might hurt more fuel mileage and utility than it helps.
- Bulky - Might not even have room to put them in.
So, obviously, to keep costs, weight, and bulk down to a minimum, you need to get away with the smallest battery pack you can manage.
The math on this is pretty simple, you just need to figure out and then decide how many kilowatthours you need to carry, for the project to be worthwhile.
Don't get stuck in the wishywashy "Well, it'll be partially electric...", because then you can't figure anything out or, you could justify anything. So, while yes you will have a mix of power, and while the motor might not be able to push the truck and pull the trailer, decide on what amount of pure electric range you want to have. You don't have to use it all electric, but you should know how much range you want out of pure electric equivalent.
Then once you have range, look at your gas mileage, and start to ballpark convert that to how many kilowatthours that will be. On a vehicle that size, you'll need perhaps 350-500 kwh per mile at highway speeds, and, double that towing a trailer (because towing a trailer cuts your fuel economy in half, expect it to do the same with electric). That will be how much battery you need to buy. Then there's the issue of, how easily can you divide them up to get a small enough pack to a high enough voltage. For example, if you're using Tesla modules, forget it. They're so large, you'll have to use most of a Tesla pack just to reach a useful voltage. You won't have money, weight, or room to fit them.
To get the most out of your battery pack, I recommend not planning electric to go more than your daily average.
Every day you haul around extra battery capacity that you didn't need is extra cost you spent and lower efficiency. Since you're keeping the engine you have zero range anxiety, so, be super efficient with the smallest pack that you'd be about guaranteed to use up in a day. You said 40-50 miles average, and at most 100, so I say plan on 30-40 miles electric. Any extra is less and less efficient. You're keen on 100 miles, but, I think that's foolish.
Let's say best case scenario you're hoping for 30 miles, and you could get away with 400 watt-hours per mile without the trailer. That's a 12kwh battery pack. Or, a 24kwh battery pack if you're towing the trailer pretty much daily.
Let's say you're using Tesla modules, just because you've already looked at them. A module is $3000 you say, weighs 55lbs, and has 5.3kwh. So, to reach you minimal needs, something in the ballpark of 2-5 of them makes sense.
That would be 110-275lbs (not bad, you could manage that), $6,000-15,000, and be, well, the size of 2-5 of them (probably something you could hide around the frame somewhere). Let's ignore that the voltage is uselessly low for now.
Just for giggles, let's say you want that 100 mile range. Now you're looking at 40-80kwh pack. 8-16 modules (where would they go?), 440-880lbs, and $24,000-$48,000. And according to you, most days you won't even use it. See why 100 mile goal is foolish?
Let's just stop right there at 30 miles.
Do you want to spend $6000-$15000 to get 30 miles of range a day out of electric power?
Well, you say you get 24mpg without the trailer, and about half that with the trailer. You pay $4.55/gallon.
So that's $0.19/mile without the trailer, or $0.38/mile with the trailer.
You're avoiding 30 miles of range by installing electric, so,
that saves you every day: $5.70 - $11.40.
With just a battery cost (let alone motor/inverter/all other systems and time spent installing it) for $6,000-$15000, how long would it take you to pay that off?
1052-1315 days. 3-4 years.
Let's be silly and go back to that 100 mile range again for a moment. You'd be carrying 100 miles worth of batteries, but only using 40-50 miles of them per day (let's say 50 to be generous). So, $9.50-$19/day. 2526 days. 7 years. Oh, and all that extra weight would probably hurt your gas mileage so much by increasing your rolling resistance (and hurt your suspension and require other truck repairs sooner), that you might not even break even, ever. The same money just invested in an index fund would probably gain you more every day, and you wouldn't have to do any mechanical work.
...
Since you were basing this whole build on getting an entire Tesla battery pack for $3000... does this new reality slow your enthusiasm for a hybrid conversion?
As a basic observation, almost no one has done this before. So either you have a revolutionary idea, or, there are practical, reality-based reasons no one does this even in a community of people who like to goof off and tinker and make stupid projects.
...
Also, this being your only vehicle, which you plan to convert... overnight? On a weekend and be rolling for Monday? ... I know nothing of your mechanical abilities so I won't say it can't be done, but, I'd say even for a very well planned install that is highly ambitious of you. Car projects are highly ambitious enough for people who are doing an extra vehicle.
...
Anyway, that should ground your expectations a bit and help steer any advice anyone could give you here on out. What kinds of choices do you want to make based on what you now know?
