new guy here.... battery chacharging questions

1.5 or 5 watts will not keep up with the parasitic draws of the trailer if the batteries are left connected to the trailer. 5 watts will not keep up with the self-discharge rate of the batteries if the batteries are disconnected.

I'd suggest to get something around at least 100 watts and a charge controller. With that, you'll have something meaningful in the way of solar power. If you check pricing, a 100W module will not be that much more than a 25W module.