My Heep gets about 18 mpg on a good day but it's got no guts. Anything I mounted on the underside would have to stand up to regular rock-bashings. I'm not sure what else I could do to improve the aero. I've heard that a steady-running diesel is about the most efficient (available) motor you could use to charge batteries. The minimum power/torque for the diesel would be what it would take to push the brick down the highway and reasonably recharge the battery pack. The dinky 4 cyl. I have in my Heep now weighs about 600 lbs, so the setup wouldn't be adding too much weight. Or would it?
Consider a regular car battery weighs around 50 or more pounds. You are going to need several of them. You can use lighter batteries, but will pay more.
The current Prius has 1.8L engine that likely weights in the range of yours. It also has a 100lb battery, alternator, and 2 motor/generators. You thinking of using a system that uses just EV for motive force, so you'll be able to downsize the engine, but that will also mean upsizing the motor.
Have you looked into modifying your existing engine? Guy I went to HS with had a mid 80's Jeep. IIRC, it was an inline 6 with carb, with mileage range from 6 to 20 mpg. A compete engine swap will be cheaper than your original plan. The battery alone will easily cost a $1000. Likely more, and those you can get at Wal-mart.
Uhg. Now I'm thinking about going to a hydraulic transmission w/ accumulators. I might be able to afford used hydraulic stuff.
Trollbait:the gutless wonder Iron Duke is the most efficient carburated stock Jeep engine available. I've got a feeling that any other gasoline I-4 I could put in a CJ-5 would get about the same mileage. Fuel injection might increase mileage by 1-4 mpg but the expense and hassles would more than offset the savings as far as I'm concerned.
I should mention that when I go wheeling, I go way, way far away into the wilderness. I try to keep the Heep as simple as possible in case I have to conduct emergency field repairs.
Possible mods on present engine:
Switching to electric radiator fan. But it violates the K.I.S.S. principle and I've already got a very light-weight durable plastic fan stuck to the water pump.
Hot air intake: wouldn't be too hard to do but I've studied the posts and I'm not convinced it works. Consider counterclaims about cold-air intakes.
Put a turbine head on my alternator and stick it in the exhaust pipe: Would probably actually be pretty reliable IF you solved problems of mounting, backpressure, finding the right turbine head (turbocharger wheel might be too delicate to handle alternator load), and alternator-cooling.
Turbocharge for mileage rather than power: heard this can be done but violates K.I.S.S. (although not egregiously 'cause if the turbo broke, you could just pull it off and run the engine stock) and the cheap Iron Puke engine will accept about 4 psi of boost max before it blows itself apart.