Solar Panel to Maintain Battery

With 15 watts you still need a battery disconnect to prevent discharge.

We keep 160 watts of solar hooked up when parked, which is a fourth of our current total, keeping 14 (10 house & 4 starting)batteries charged and a small Whynter fridge/freezer running full time. We'll be adding 8 more batteries soon. Where we are, in the summer 160 watts provides a slight surplus, in the winter it's just about even on sunny days, and a slight deficit when cloudy. I'd love to get more panels somewhere along the way. The more watts you can get when in storage, the less you'll have to be concerned about your batteries.

My only recommendation is to wire the panel so that it bypasses the battery cutoff switch, and make sure the cutoff switch is off when the rv is in storage. A 15 watt panel probably won't keep up with your parasitic losses with the battery connected, depending of course on your particular rig.

That should work fine, we use a 35 watt and 10 amp charge controller to keep the 2nd car battery up for 6 to 8 months.