Well that's the second part of the mystery.
1. Why wasn't it kept charged regardless of the USE/STORE switch position?
2. What could be draining it?
Your dash radio does have a constant 12 VDC line to maintain memory settings that goes to the chassis battery. I'm not sure what other constant 12 VDC load would come off the chassis battery.
Ted & Melinda
2016 ACE 27.1
2016 Chevy Sonic Toad
2020 Chevy Colorado Z71 Trail Runner Alternate Toad