It is possible that something got left on from your last trip that drained the battery?
Good question, my very smart spouse asked the same question.
If something was left on that did drain the battery, and then now all are are off (because I verified with the van engine on).
Would something that may have previously drained the battery completely kill the battery from ever recharging?
Lets try something different and try disconnecting the extra battery from the isolator. Then retake your voltage measurements.
Can I do this measurement by disconnecting under-hood circuit breaker from home battery, and then measure voltage from terminal on under-seat circuit breaker against the van body as ground?
The battery I purchased in 2009 was a Fullriver DC200-12, BCI Group: 4D