what is the best auxilary battery for a 1993 eurovan westfalia?

I would use an Optima with the largest reserve capacity you can find. Your local auto parts house sells them, I would also recommend staying away from lead acid batteries.

Well, on my 1993 Eurovan I opted for 2 x 31D Gels. Why? Gel far exceed AGM's for longevity and, with proper care, the Gels will last 10 yrs. or more. I've done it on our sailboat by installing a smart charge system on both solar and alternator charging. This limits the charge voltage to 14.2V. and feeds the batteries in a "stepped" regime. All boaters that I know that have installed AGM's are replacing them after 5 years and lead acid batteries are too messy and emit H gas when boiling - not for me.