MFL wrote:
If you are looking for a single marine/rv type 12v, I'd suggest a group 27, or 31, if it fits. My dealer recommends the interstate brand, and mine is still in use, since Aug 2012.
IMO, the secret to longevity is to not discharge these battery types to near 50% often. Mine is connected to shore power, or gen, most times, other than when towing.
Jerry
I had two Kirkland size 27's in my previous fifth wheel that were over 10 years old when I sold that fifth wheel . I agree, mine were never allowed to be very discharged ,they ran whatever including the furnace many nights without shore power , but were charged right back up every morning by a generator .
The fifth wheel when not being used or towed was plugged in 24/7, water level check frequently . IMO any quality 27 or 31 will work, Interstate is probably as good as any , I believe Costco sells the Interstate .
On a further note ,I have to Everstarts from Walmart in my 2014 Ram 3500 that replaced the OE's that have performed well. If I was in the market for just one RV battery I wouldn't hesitated to try one of the Everstart Deep cycle or AGM for my fifth wheel. My present fifth wheel I run two quality 6 volt , which IMO is the way to go ,but apparently the OP does not have room for two batteries .