Joining a bunch of SLAs (Sealed Lead Acid) batteries together in SERIES will work just fine -- I would suggest best for lower/intermittent current draws like lights, furnace fan, stereo, etc.  I would avoid using for 12V heater on stock Westy fridge because of the current draw, for example.  This setup has the advantage the you can customize the space and capacity of your house battery bank, but you will have to do some wiring.  (Make sure you are certain you're wiring things up right or there will be smoke!)

I did something similar with a previous westy - I found a 12V deep cycle SLA, around 35 a-h as I recall, normally used in electric wheelchairs that fit very nicely in <1/2 the space under the driver's seat.  Worked well for my modest needs and I wasn't stressed about discharging the starting battery.

tabe johnson/90MV