How many watts.. Depends, What do you have in the rig.. TV not many, Microwave, more, Air Conditioner, LOTS on a pop up, Most generator user manuals come with some examples, Add 'em up and that's what you need
The following URL, is a download link, It is a PDF of an owenr's manual for a Xantrex Prosine 2.0 inverter/charger
(Which by the way is not a bad choice if you want one of those)
http://www.xantrex.com/web/id/238/docserve.asp
Take a look at it, there is a chart that lists common power loads