Sportsmobile makes a good argument that the main reason to own a generator is so campers can run the air conditioner without shore power.
Sportsmobile Custom Camper Vans - Generators
The need therefore depends entirely on how the motorhome will be used.
It seems to me that if you need a generator anyway, then the economic justification for solar is significantly reduced. On the other hand if the air conditioner will mostly be run from campground electric power, making the generator a luxury more than a necessity, then the cost of a generator can be applied towards solar and additional battery bank capacity.
The trend with smaller Class Bs that are custom designed (i.e. -- owner has input) seems to be towards deleting the generator in favor of larger battery bank, some solar, and engine charging off alternator. But Class Bs tend to get driven daily, so it makes more sense than for motorhomes that stay in one place (and without shore power) for long periods.
Perhaps the original owner of this motorhome felt he/she didn't need air conditioning very often in Canada and therefore didn't need a generator. In any case the new owner wants a permanent generator.
I would first try to identify under what conditions I expect to need the generator, and then go from there. I wouldn't necessarily oversize the generator because it adds initial cost, weight to RV, and consumes more fuel at reduced partial load.
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