That was my initial thoughts when I was mulling this whole idea over several years ago, Orcadrvr. I think that's about the only way it would work.
The diesel engine would be scaled up to keep the batteries charged and supply the motor during those peak times...from a standing start, up hills, etc. It doesn't take much energy to push a RV down the road in calm winds so once it's moving on flat ground, the engine would keep running to top off the battery. After that it could shut off quite often.
While coasting, or heading downhill, regen would kick in to both slow the vehicle, (one of the benefits of a DC motor that while supplying regen current it acts to brake the vehicle) and to use that regen current to charge the battery.
Another benefit is that simple resistors (large ones of course in this app) and large contactors could do all your braking. A DC motor, with its leads shorted is impossible to turn. Which makes it a great brake. If modulated on the downhill, you could even pull to a stop using just the DC motor. This means lighter and less expensive service brakes.
So much fun.
