Assuming everything that is possible to turn off is "off", and the rig is in the sun most days (not under roof or trees), I would guess 25-30 watts would do it. That would produce about 1.5 amps in full sun, a reasonable "trickle" charge for the batteries. In the Northern US during the winter, when sun is lower and skies often cloudy, I would want more, maybe 50-80 watts.
There are many variables, so this is strictly a WAG.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL