A Volt-E Bag charging question

Howdy y'all.

Love my Volt. Have only been camping in it twice, and both times, plugged in to 120v, aftermarket charger set to 13 amps, interior swapped the charge mode up to 12 I think the option is on the console.

Both times, mild climate use (keeping it at 63 when it was 72-50 outside), charge ran down to the point the engine kicked on in the wee hours. Did the juggle of turn off, plug in, turn on, shift to neutral with parking brake on and let it run.

I'll try with a shifter clamp next time, but mines a 2014 so I don't know if that's needed. I want to figure this out before I try keeping it at 60 when it's below freezing or the real golden goose, keeping it in the 70s when it's 90+ outside.

Should I be doing something else, some other trick? TYIA!