Nice... Good build in progress.
I've run a double battery on all my previous rigs. Still debating on wether or not to put it in the 100.
Up to now I've had good luck with this as a back-up
Amazon.com: Antigravity Batteries AG-XP-10 Multi-Function Power Supply and Jump Starter: Automotive
Basically when I do a trip where I drive all day and set up camp in the afternoon and continue on the trail the next day, I've seen one battery is enough as long as the fridge is kept on a lower setting for the night.
When I stay longer than one day at the same spot, I just take my small Honda generator with me. This thing is VERY quiet and with a 100 ft extension cord, we set it far away as to forget it is even there.
https://www.amazon.com/Honda-EU1000...r=1-1-catcorr&keywords=honda+generator+eu1000
The only time I've needed a jump start from the XP-10 was at a campground where we where parked for three nights straight and I had to turn the generator off from 10:00pm to 7:00am quiet time. The fridge was kept on low with the car battery but I never started the engine once in three days... The battery had 11.8 AMP when I tried to start the truck, so for three nights powering the fridge, it is not that bad (and I have a 14 year old ARB Fridge, which does need more juice than the newer models).
I'll wait and see how the summer camping season develops to make a final decision on the second battery as TX gets very hot and that might have the fridge working overtime inclusive at night.