Hi Larry,
For me, at least, while traveling/camping all winter,
sometimes in remote areas for days on end without
motive to run the engine, I use my little Solarex photvoltaic
panel (11" x 18" x 1/8" thick) to top off both my starting
battery & my aux battery.  It is also useful, when van is
parked for days under shade trees, to set up the panel in
full sun & plug in the little charger that does my small batteries
used for powering book lites, flashlites, etc.
I used this system last winter for 6 months traveling Mexico,
& was never without all the "power" I needed for anything.
This year, I've added a small inverter that will accept a proprietary
Canon charger for my digican &--best of all--my electric coffee grinder!
Phaedra
'85 Westy-Wolf