Agreed. 5 watt panels are really too wimpy for RV charging, but if all you're planning on doing is keeping your battery at a decent float voltage, I guess it could work OK. With that small amount of power, you might not even need a charge controller. I don't see a panel that size cooking a large battery. You might even lose more than you gain, over time. You'd want to keep an eye on it for a week or so. The good thing is this approach is super cheap. Amazon has a ton of 20 dollar 5 watters.. I don't have any recommendations as to which one.
Between the smoke alarm / monoxide detector use, 'parasitic drain' and natural battery losses, not sure if it would keep up. Requires more math than I can do at this point in the week
Edit - just remembered something.. that 5 watt panel, on a good day, is only going to give you about a 0.5 to 1 amp hour to your battery. Not sure how big your battery is, but I'm guessing you are losing more than that with the battery just sitting there.