Why do you want one? The propane and battery systems in your RV will operate everything except the microwave and AC unit. Eventually, of course, batteries need to be recharged but your tow vehicle will do that if your traveling around frequently. If you intend to stay in one place for a long time a solar system is another option for recharging batteries.
In fact, with a large enough battery bank and an inverter you could even run your microwave and other appliances without a generator.
If you want a quick, easy way to operate high powered AC appliances then a gen. is probably a good idea. If you want to operate the air conditioner when you have no hook-up it's a requirement. Otherwise, it's a good way to make noise, use gas, and cost money.