The problem with relying on a very small solar panel is that, while it might maintain four large batteries with adequate sun exposure, if you drop in to check on the trailer and run some stuff, say the furnace, while you are doing things in there, you might deplete the batteries further than that dinky "maintainer" can deal with. It is intended to keep one or two fully charged batteries topped up, NOT recharge four of them... it only puts out 1.5 amps in the best of circumstances.
I would suggest a 100 to 120 watt folding (suitcase) unit, so that it will recharge during storage, as well as be useful on camping trips. The larger size will also make up for degraded charging due to the grey days and much shorter hours of winter, a partially shaded panel, dirty glass, etc. Bigger is always better in this situation - it gives you some reserve capacity.
Portable solar for RVs
John Davies
Spokane WA