Problem solved - I learned that either myself or a camper repair shop incorrectly wired the two 6-volt batteries together and there was a somewhat hidden blown in-line fuse. So I replaced the 30-amp fuse, correctly connected the batteries, and hopefully with proper electrolyte levels, the batteries will hold a charge -which seems the case after a few days. The blown fuse explains why there was no 12-volt power from the truck to the trailer and maybe why the batteries were not charging from the truck or shore power. I'm sure the crossed battery
cables and low battery
water didn't help much either.
Thanks for the help! I'll also be purchasing a smart charger soon.