Just put a 2 battery setup in my Outback 25RSS yesterday, planning a boondocking trip later today to RMNP to Glacier Basin CG. Interested to see how they perform.
Everlast Maxx 29DC, hooked up in parallel. 1 of them replaced a 24 on a straight across swap (defective battery under warranty) and I bought the 2nd one. Did a lot of reading here about batteries and figured for my purposes, this would work fine for us. They were slightly larger than the battery trays, but I've got them bungee'd down real tight, so they are not moving.