You'll need a place to put the controller (switches solar charging on and off and allows you to monitor the results); you'll need cables to both the solar panels and the batteries, where ever they are.... hard (emotionally) part is getting cables through the roof - one popular method for trailers with roof refrigerator vents is to run the cables down that way. Barring that, you'll need a through-roof fitting.
I'd read the AM solar installation guide - it's pretty good, and covers what you need to know. If you can have a raceway to install cabling later, that's perfect.
AM Solar's Support Pages PDF Downloads
I bought my kit from them and was quite happy... it was worth the extra $ to get all the right parts, UV-rated ship's cabling, etc.