Did you think about replacing the lead acid 12 v with a LiFePO4 battery? Should fit, and because of it's chemistry you can safely discharge them to near zero SOC nearly doubling your usable capacity, you also save on weight, they recharge much faster and are truly maintenance free and have a life of 3000 - 5000 cycles. Upfront cost is more but have a 10 year warranty.
I replaced my 4 GC2 with 2 Battleborns 6 months ago and glad I did. No watering, corrosion, acid fumes on charging, less run time on the genset because they charge faster.
2014 Sportscoach Cross Country, 361BH