Not a surprise to me, but that's because I spent a decade in the Rockies with a 5vz-fe in my Toyota T100, only moved 3 times in those years, all with the T and an enclosed trailer (loaded to the freaking gills, I was just over 3500lb here, leaving GA for WY to visit my folks, then down into the 4-corners area for my wifes new job):



The Worst fuel economy I saw, into headwinds in SD, was 12mpg.

A couple things come to mind... first is that your MPG is way low if you don't normally see 20mpg (when not towing/driving normally). It's not like you're on massive tires with a ton of extra weight there... sounds like some maintenance might be needed. O2 sensors, cleaning the MAF, new air filter, plugs and wires, alignment, checking the brakes for drag, fluids, etc... it can all add up. Only typing this incase you don't realize that's a low normal MPG.

The next is a question... are you charging the house battery in the camper from the tow vehicle? Given the discharged house battery, low power and low voltage while driving, I'm thinking that you were running some electrical load in the trailer, it eventually depleted the house battery and started drawing on the 4R start battery which in turn caused the alternator to load and rob HP from the rear wheels. I don't think a vent fan is capable of doing that kind of damage (I'm assuming it's like my MaxxAir fan on my campervan, I can run that for nearly a day on high from the start battery before getting concerned about being able to start... when running, there's no noticeable change ~ it simply doesn't pull much energy). This feels more like an inverter was left on with something running to be able to pull down a 70amp alternator.

BTW, jealous of your trip. I'm a WY native and love it in the Rockies, but my wife is from GA and women seem to have to be near their parents. My trips are way too infrequent and short due to the distance. I spent a couple weeks in MT this June for my 15th anniversary though: