18 to 30 watt panels are a waste of money. It doesn't cost that much more to get 75 or 100 watts. Keep in mind that unless your using an MPPT charge controller you will likely only get about 80% of the output of the panel.
Your battery sounds really small, what is it? The most basic of all RV batteries is the group 24 which has about 75 Ah. For maximum battery life don't discharge yours more than 50%.
I would at least start with a 50 watt panel. Then, if it's not enough, add a second one.