A lot of RVs already use that roof space for solar panels and battery charging. For the house batteries.

An electric car makes sense; the majority of the cars in the United States are used to drive a double-digit number of miles in a day, are not expected to provide a significant amount of energy for anything else, and they sit parked most of the time. You can run that mission on batteries.

RVs are expected to drive hundreds if not thousands of miles, then be a house for a week, and then drive hundreds if not thousands of miles back. A large RV will be equipped with a main engine for highway propulsion, an APU for recharging batteries or running HVAC, a bank of deep cycle batteries for power when no engines are running, and a bottle or two of propane to run the stove and refrigerator. Solar panels on the roof are often used to extend the parked "boondocking" endurance, as running the APU (or in some RVs especially those that aren't built on tour bus chassis, the main engine) consumes motor fuel, making it possible to strand yourself.

That does assume "boondocking" or parking somewhere without utilities. A lot of RVs are driven from socket to socket and all of the house systems are run from 240V mains electricity and the batteries may see mere minutes of use, but even then while traveling you want the fridge to stay cold and the lights to work while you're parked at a rest stop in Oklahoma having lunch on your way to the Grand Canyon.

An RV is perhaps the last vehicle I want to fully electrify.