Doable, but not cheap. Particularly if you buy a new motor & controller. Batteries will either weigh you down or cost a lot.
Used motor, modest controller won't necessarily break the bank. You just have to find one. If its just short bursts, you could probably cheap out on the batteries too (marine deep cycle batteries, rather than golf batteries or lithium).
Expect it to cost at least a grand...and that's cheaping out.
It would only be used during take off and acceleration. The intent is to use the electric motor to provide additional torque and reduce the load to the engine.
Vehicle - RV with a 7.5L 460 Gas Engine.
Not even a daily driver? It'll never pay for itself. Unless, maybe, you use the battery pack as a power supply for your RV.
Something that big will need a bigger motor than what I gave you $$ figures for. That would only get you ~10hp. You'd want at least triple that. IE a full sized EV motor. And the batteries to go with it. Find someone selling an EV converted vehicle for cheap if you want to get affordable parts. Unless you're lucky, you'll probably still need to buy batteries to go with it. "EVTradingpost.com" might be of use to you, if you're still interested in the idea.
There might be cheaper ways of attaining what you want.
I'd probably lean towards a supercharger or low-pressure turbo. A small turbo adding 6psi of boost would be easy to engineer and probably add substantial torque at low revs... and probably cost a fraction of a motor & controller.
All that roof space on the RV could be used for solar panels to help charge the batteries. But as mentioned above, an electric motor powerful enough to make a difference on an RV sized vehicle is going to be expensive. You might actually be better off replacing the gasoline engine with a turbo-diesel.