Check the solar panel link on this page.

http://my.tbaytel.net/guskers/vanagon.html

This charges the aux battery, and is switchable to charge both.  It works.
On sunny days, the charge controller is often showing "green", meaning the
battery is completely charged.  Total cost about $200 Canadian.

If you use a small charger with no working diode or controller, you are
wasting your cash.  In cold climates (-21C last night), with shorter winter
days, a properly set up small dash top unit will likely not even cover
standing voltage loss to the battery.