I bought a 70W Solar Cynergy panel and a Morningstar SG-4 controller from solarblvd, and I kept it portable. I wired the two using a short extension cord I cut in half, then plugged the two together, or added an extension cord in between as needed, at the campsite. The other end of the controller I wired to a 7 pin Bargman, and I can plug the trailer pigtail into it... presto, battery gets charged. I kept the box that the panel was shipped in (foam insert and all) and I use that box to store the panel when not in use. It traveled behind my driver's seat.
You could put together a similar setup for about $130 or so:
100 watt panel6A controller