While keeping the 5th wheel at a storage yard, and starting the storage time with a fully charged deep cycle battery, you really don't need much to keep the battery topped off. A 10 or 20 watt panel would be plenty. $35 or $40 plus the necessary connector should do it. And at that cost, if it sprouts legs and walks off you aren't out much. With my previous TT, which I parked in the back yard, I routinely left a 70W panel and Morningstar controller hooked to it, but that was overkill (I bought the panel to take boondocking, so I used what I had).
If you were only leaving it in storage for a month or so at a time in moderate temps, the disconnect switch (plus washing the top of the battery) might be enough to keep the battery from getting low enough to be damaged.
