Solar trickle charger for chassis battery......Good or bad?

Install the big solar system and a good quality controller (that will cut out when the battery bank is charged) and call it done. No need for a second system.

If you can't get the big system installed, a 20w should be pretty harmless. They typically don't include a controller because it's not putting out enough juice to boil off the battery fluid. Of course a small charge controller is pretty cheap so if you feel like you want extra insurance, no real harm.