I found that one 100 watt panel is sufficient for my particular needs. I don't run a microwave oven or have any other large electrical loads.
Being able to move the panel to avoid shade is a HUGE benefit - allows one panel to produce the same amount of power as a dozen roof-mounted panels in partial shade.
My 17' Casita isn't factory-equipped with a solar power connector, so I just fed solar power to the house battery through the 7-pin hitch connector instead:
The panel is connected to this "solar charge box" through approx. 50 feet of wire, allowing the panel to located some distance away from the trailer.
Total cost of the "charge box" with the Renogy charge controller, 7-pin receptacle, wire, etc. is maybe $60.