You mentioned your goal was to just "top off" the batteries while in storage. Likely, when you are driving home from your trip the batteries will be getting charged. So really you probably only need about 100-200w max.
Suggestion: Look for USED panels on craigslist or facebook marketplace. You might be able to find healthy 250w panels for less than $100. An inexpensive PWM controller is probably all you need (~$25) if you are putting up 200-250w.
You probably only need 100w, but if you are buying used you can likely find 250w for a great price. When I was adding solar onto my RV I only was planning on putting up 250w. But I was able to get the 250w panels (used) for $35 each, so I bumped it up to a 500w system which is overkill.
Again....this is assuming you are mostly interested in keeping your batteries fully charged while in storage.
Good luck!
Chris
