The generator, if powered by a separate engine will have several other factors.
As a small engine, it will not be required to meet the same "clean air standards" a vehicles primary ICE unit would meet.
It adds another factor of the same weight it was supposed to initially be eliminating, along with the fuels etc.
If the initial goal was to be going "all green" and eliminating carbon based fuels (not counting what charges the batteries at home) then the carriage of a separate engine/generator seems to defeat the original purpose.
No matter if it is a 5 hp or a 200 hp engine, the vehicle will still require a similar amount of energy to move it. The little generator just takes a short period of movement, and spends more hours at lower per hour energy cost creating the energy, it still requires the same total amount. It is not a perpetual motion machine, it does not get something from nothing.
As to what wears out, Heat and friction are the factors. Higher pressures and higher RPM's are primary (not the only ) contributors.
And as to HP vs size consider
"The current, third-generation IndyCar formula was introduced in 2012. The engines are now fuel-efficient DOHC 2.2-liter twin-turbo V6 with four-stroke piston Otto cycle developing an estimated 550–750 hp depending on the level of boost used and no inter-cooling systems."
More HP than the 6.8 driving most class A motorhomes, Would you want to try using one long term in that usage? How long do you think it would last?
Hybrids and EV's have a place, where their short range does not overly limit them, but when you add systems to an already heavy vehicle because of the batteries to get past the range limits, there are just too many factors working against them.
