The first question is, what size is your power cord to the RV? Most likely it is a 30A cord so no matter what you do, that is the most power that you can use so no generator will gain you anything with more power available than that.
Assuming that you have a 30A cord, to determine the number of watts that your maximum will be, multiply the amps times the voltage.
30A X 120V (nominal shore power voltage) = 3,600 watts or 3.6KW
This means that the most you can ever supply to your RV is 3.6 KW of power, no matter what you try to use. This is the limit built into the RV by design and to buy more than that would serve no purpose. It would be reasonable to then buy a 4KW generator just to give you some room for the starting surge currents of some things since it is possible to exceed the basic limit for very brief periods when an appliance starts, but I would not consider anything larger as it would be a waste of money.
The most common size generator which is readily available in that size range is 3KW. It seems that everyone makes one of those and they are probably the most cost effective. To determine where you stand in terms of power to the RV, just figure backward to get the current it can supplyl
3KW = 3000 watts / 120V = 25A So the maximum current that a 3KW generator can supply to your RV is 25A.
For most users that is sufficient for most use. It will supply your air conditioner (typically drawing 13-14A) and still leave plenty to also operate the majority of appliances as long as you understand that you do have to use some care in what items you operate at the same time. It will not supply two air conditioners at the same time in most cases.
If you have a 50A power cord, the same math applies. 50A X 120V = 6000 watts = 6 KW
This means that to supply the maximum power for a 50A RV you will need either one 6KW generator, or you can get two of the 3KW sets which have the ability to operate in parallel. This also becomes much more complicated to get started, since you will then also need special wiring to connect the cord to two different generators, but it is done and is becoming more frequent. Let us know which need applies to you and more advice can be given.
Um, a 50amp power cord has 2 circuits (180 degrees out of phase) so the total power it can handle is 100A which would be 12KW. However, most 50 amp RVs don't use all of that, so a 6 to 8KW is often adequate