If you look around, you will find that some motor homes come through with a generator as standard equipment. Granted you will pay more for the coach, but it will be one of the better coaches.
If it is an option and you have it installed at the factory, it might cost 3 to 4 thousand dollars. It will be added to the list price. Nobody pays list prices. If you get a loan, the extra cost of the gennie will be spread out over the loan period. If the dealer installs it there is no cost cutting. Probably 5 to 6 grand, if not more. You can always take it out when you sell.
Either way, they most likely will put in an Onan 5000 watt generator. Which is a very good unit. You may upgrade for more $ to an 8000 watt.
If you buy used, with a generator in it, The owner just wants to get the thing sold. He won't ask more for a generator. Again asking price and selling price, two different numbers. Just make sure, buying used, everything is in working condition.
Just an example. We looked at a 40 ft motor home at the Rockland show. Listed for $269,000. We got them down to $169. It had the 8000 watt Onan, diesel in it. The cost of the gennie got lost. We put a down payment on it, but the economy put the kibosh on that deal. I'm not saying you will get a deal like that, but they are out there.
I want to clear things up on my Gentron. It's 4000 watts, 3500 peak. That is plenty for the trailer. Champion, also makes a 4000 watt gennie. Cheaper than the Gentron. My son bought one. It doesn't come with a 30 amp outlet, needs an adapter, or electric start. It looks like the same generator, other than that.
Just giving you some more options to kick around. Check out the Rockland show. It should be coming up. We go every year just to eat our heart out. LOL. There is also a show in Hersey, Pa. The manufacturers are there. A very good deal could be had there. You can order what you want. It's huge. Two days to see everything. Good luck. Bob