RE: Aux Batteries
I've stayed out of the various recent battery threads because I don't have
anything to contribute beyond what was being said.  But Dennis just made a
good point that I want to second based on my experience over the last 10+
years, first with an '85 Westy with a Surepower isolator and the last four
plus years with a '90 Westy and the basic relay setup.  I've only used a
stock Group 41 lead acid starting battery for the aux battery from day one.
Simple reasons:  ease of install and cheap.  I went through 2 1/2 batteries
in ten years in the '85, one so far (replaced the battery that came with) on
the '90.  For the price of a Group 41 at Costco (no longer carrried in my
local store) or at Wallyworld I can go through three or four batteries
before I meet the cost of an Optima yellow top and don't have the hassle of
mounting the Optima.  I play the stereo a couple of hours a day, run a
laptop an hour a day or so (process pictures and watching the news on TV)
and have gone to all LED bulbs internally (stock lighting plus three other
lights I've added).  I just came in last night from six days and nights
parked without starting the Westy and when I packed up to leave there was
still plenty of juice left in the aux battery when I checked the meter I've
installed just to see.
So if you are a little miserly with the electric usage the cheapest and
easiest way to go is the stock Group 41 starter battery under the drivers
seat.  KISS

Footnote:  If I were fulltimeing or doing a lot of extended camping I'd pick
up a little Honda generator, fill my spare gas can, fire up, plug in, sit
back and relax.