what size generator ?
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)