Hi sylvia, you should be thinking of at least a 3000 to 4000 watt generator, if thinking od installed in the 5er then look at Onan, Honda, Generac(my preference in that order) and yes any rv gen should fit, if your thinking portable generators you'll still want 3 to 4000 watts and Honda, Yamaha make the best, quietest, and those would work best in the bed of the truck. :laugh: :bleh: :approve:
PS are you thinking of doing a lot of boondocking, thats camping with out hookups such as in state and fed campgrounds a generator is nice, if you'll be using full service CG's with electical hookups at the least, you may not want to invest the money in a generator. Generators are spendy and if not used regularly can be troublesome.
If it has a generator compartment, you will probably have to use a built in generator. Stand alone generators are usually cheaper and sometimes quieter (the Honda EU series), but are an annoyance to use since you have to get them out and set them up, and they tend to 'freak out' if run in an enclosed space. Back of the truck can be a good place for a generator from a conveniance standpoint (that's where mine is), but it was expensive and a lot of work to protect it from weather and unauthorised access.
As to the size, how many/how big AC units do you have? If 1 unit, you might be able to go as small as 3K watts although that won't run all ACs out there; 4K is more common and often a better choice. If 2 AC units, you are looking at over 6K watts. Another way of looking at it, is what is the current your trailer can handle? If 30 Amps, then anything over 4K watts is wasted, but if you are 50 Amps you will probably want 7K to 8K watts (or a bit more under some circumstances)
I full timed in a 35' class A with a 9 KW Onan for 3-/12 yrs and I am a nut about preventative maintenance. It is my opinion that gennys are far more troublesome vs time used than any other component in a rig. If I buy another unit, I will buy it without a genny and equip it with solar panels and pull out a Honda 2000 genny if I need to goose it for the microwave when remote camping. These Hondas are very quiet and reliable and inexpensive. There is a guy in N. FL who does this sort of thing and a few more in AZ so whereever you are, competent advice and installers are available.