What to use to keep battery charged while sitting?

To the OP, slightly off topic but my Atwood helium fridge works great on propane, are you sure there's not another specific issue with yours?

I use a small 2 watt solar charger mounted on the top of my propane tank cover. it puts out 500Ma which is enough for a trickle charge. For what you propose I would definitely go at least with an 80 watt panel and a good charge controller.

I'd suggest a simple solar system. Solar will do the best job of maintaining your batteries.

take the battery home with you and put it on a battery tender.

The only thing your missing is solar, it's the best way to maintain batteries without shore power. If you do any boondocking your going to want to keep your batteries topped up.

You say it's stored in a "storage yard". Is the TT covered when in storage? If so, then solar isn't going to help you keep the battery charged while in storage. In that case, I would look into the trickle charger.

Will the storage yard let you plug into to shore power for a couple days before you take it out of storage? If so, you could stop by on day 1 and plug it in to shore power, then stop by on day 2 and start the fridge, then on day 3 go RVing. That will charge the battery and get the fridge cold before your trip. And if all you're doing is charging the battery and turning on the fridge, you're not using a lot of the storage yard's electricity.

Hi,

Definitely consider solar.

100w solar and a good controller.
JMHO

Do you have access to shore power while in storage? If not, then solar is your best bet.