Question: Generator For An Old RV

Not really enough information in your question. First thing we need to know is What do You Want to do With It? If you don't need to run air conditioning, you don't need nearly the power that you do for A/C. If you want to run it a lot, not have your neighbors mad at you, maybe even mad at yourself, most of the "jobsite genny's" are too loud. Some genny's in the class you noted have TWO 120VAC windings. Going across both lets you get full rated watts BUT at 240VAC, not what you want. Either of the 120VAC windings will give you only half the rated wattage. A true RV GenSet is big, heavy, and expensive. When we had a 1984 Class C, I bought a used ONAN 4000W for $800 and was glad to get it for that price. It weighed 250# and was pretty tall. But it was a "whole house" genny. It could power everything I could run from a 30A shore tie pedestal. That was because it had ONE 120VAC winding, so the full 4000W was available to start an A/C.
Many of us here look to a portable Honda "inverter" generator, 2000W. VERY quiet and will start a 13800BTU A/C. Might be available used on places like Craigslist. Some models can be "paralled" for 4000W.
Some have mounted "portable" genny's in their RV's but it's a complex job to get it safe and cooled and quiet and fueled.
Coach like yours probably had ONAN or Kohler or Generac 4KW. Big and heavy. Later models like ONAN MicroQuiet (the "KY" Series) have about the same footprint but aren't as "tall" so they'd fit your genny bay.