If you already have the 1.5watt, give it a shot and see, but be sure to utterly cut off the juice to the trailer and make sure the batteries are fully charged before you put it in storage. It may well slowly fall behind, but probably so little that you'd be fully charged again by the TV alternator by the time you get to any destination.
Anybody telling you you need 40w just to maintain state of charge is either selling solar kits or smoking something...
Such a small panel shouldn't need a charge controller. It's not capable of boiling your batteries. Actual recharging and periodic equalization would need to be done via shore power and a good charger or three stage converter/charger. And don't forget to three times a year at least top off your wet cells (assuming lead/acid)with distilled water.
Google "the 12 volt side of life" for more than you ever wanted to know about your RV 12 volt electric system.
