I know this is an "it all depends" kind of question.  But maybe some good discussion/ideas.  About 15 years ago I installed the GoWesty auxiliary battery setup which includes recharge when driving; I used a Group 41 Walmart battery, same as my Starter Battery.  Works great!  In my '85 Tintop, my camping electrical needs are minimal:  just some small interior lighting, and a small 150 W inverter which powers my laptop;  usually watch a DVD in the evening.  Also recharge phone.  I theorize that even if this "house" battery gets very weak over the years, that is fine; should still be able to power my needs.  No DC refrigerator, I use ice chest.  Eventually, the aux battery will croak and inverter probably will cease to function; and then I just get a new battery to last me another 10-15 years.  So, the question is...what is lifespan of Aux Battery?