Thank you RoyBraddy and Rustic Eagle for the good information about setting up and maintaining a 12V supply for dry camping. I used it to come up with a plan that will work for us.
We wanted a small TT, and we really like our 17Z - but it has its limitations. The 2011 17Z has a 415 pound hitch weight leaving the factory. After adding a 50 pound battery immediately behind the jack, the hitch weight is 13% when you tow it home from the dealer. Most of the storage space is in front of the axle, so after loading your gear there’s not a lot of weight tolerance near the hitch. And after I added a 2nd propane tank, there’s not a lot of room either.
Partly because of the weight issue, two separate batteries will work better for us. We don’t need the amp hours from paralleling (although it would be nice). A bigger factor is, we won’t use the 2nd battery often enough to maintain both the same. I’ll probably go with a sealed type for the spare so I can haul it in the bed of the truck. I’ll keep it charged and change it out if needed. It’s probably more than most people would want to deal with, but I don’t mind doing things like that. Also, I’m aware that in real-life situations the first one will usually shoot craps somewhere between 3 and 4 am.
I did a test in the driveway with the temperature in the low 30's using the 2 year old battery that's in there now, an Interstate SRM24 (about 80 Ah). I kept the temperature inside at 60 degrees for nine hours and the battery was at 12.3 at the end. There was no other DC draw during the nine hours.
Since the last post on this thread I got a good deal on a NOCO G26000 battery charger. It has more features than I went looking for, but it should be able to handle any battery problem I have. Next purchase: portable generator.
Right now, that’s Plan A. There is a Plan B - if we like dry camping. With some minor tray modifications, I could carry a 2nd battery and parallel them. To adjust for the weight I would haul the spare tire in the truck.
Bottom line: I think most of us would like to have Seann 45’s 12V capabilities, but it ain’t gonna happen on a 17Z.