How old are the batteries?
Might be a bad battery or cell. Might think about replacing all of them (Cant mix old with new batteries so replace all now with same battery).
I like AGM's personally but they do cost more $$.
You have a nice DP coach so you might have FOUR 6volts in your system?
2017 Axis / 25.4: Big Foot Levelers, Aims 1200 Watt inverter, Ceramic Window Tint, Full River AGM 6V batteries, Front Hellwig Sway bar / Rear Track Arm, Safe T Plus Steering Stabilizer, PD 4655 converter, Hard Wired Surge Guard, WEboost + WiFi Ranger, LED Headlights/Driving lights, TPMS, Surge tank, Tornado flush, Viair Compressor