First there is a special carrier for a motorized wheel chair, A "Cargo Carrier" is .. Just that, for carrying cargo, ANY cargo, and far as I know no law prohibits a protable generator from being CARGO running or not.
So I will list my concerns
1: Legality: I know of no law, if anyone does, please advise.
2: Security: Portable generators, Epically the kind we RVers like (Quiet Inverter types with nice clean output) are very attractive to legs, that is they tend to grow legs and walk off on you. When running the genny has to be exposed, and .. Well, if you can see it, you can steal it.
3: Exhaust fumes (if running) should not be a problem if on the back but strange things happen back there so you never know, Make sure your CO detector works.
4: The power cord. If it runs UNDER the RV, there is a chance that if you bottom out the cord can be damaged and short out,, If you run it alongside the RV,, Well I'm sure there are possible issues there too. But it needs to be secure.
I HAVE put a portable Generator (NOT a Honda) on the rear carrier and ran a cord just to the converter (Which is in the rear most compartment on this house) to charge batteries.. A Genrac 1000, alas I was storing it in a garage and it grew legs. I think I know who's legs but proving it is another matter.