I have a small sailboat with 100% LED lights, depth sounder/chart plotter, and occasional charging of vhf radios/phones/laptops via 12v plug or inverter. No running water, no heat, no fridge. Basically I'm not burning much electricity. I have a single group 27 12v battery and a 20W solar panel with PWM charge controller. We can go out for a week or 10 days and keep the battery fairly close to 100%. Before I switched to LEDs it was nip & tuck whether I could go more than a week without finding a charger somewhere.
With an RV I'd be *very* surprised if a 15 watt panel would be enough to do more than maintain ONE battery while the rig is in storage. Depending on what the draw is for your propane leak alarm and anything else that uses a trickle of power when the battery is hooked up, it might not be enough. It's definitely not enough to boondock.