Thanks for all of the info. I have a better understanding of how lithium batteries work, but between my situation and also not being 100% certain that I won't overcharge or completely discharge the battery, I'm now leaning toward AGM even though that will have it's own challenges, but I consider them to be lesser challenges. For example, I can use an extension cord from my friends house to charge the batteries if needed instead of taking the battery inside.
The LifeBlue in-battery monitoring looks promising but I'd rather wait until more people have used it and posted about it.
Adapting, as always :)