According to Dometic, the recommendations for generator size for their 13,500 BTU RV air conditioners is 3.5kw for 1 A/C and 5.0kw for 2 A/C.
You can download the manual/spec sheets here:
http://www.intechtrailers.com/images...nit-manual.pdf
There are several aftermarket "fixes" for using smaller generators, but they are, in the opinion of many, not the "best idea".....
There is a current thread here:
http://www.keystoneforums.com/forums...ad.php?t=36162 that discusses the question you ask. If you use the search button at the top of each forum webpage (in the black bolded section) and search for the term, "Generator" you will get a lot of information about generators, using them, maintaining them and limitations with some generators.
Oh, welcome to the forum. We're glad to see you here.