@immanence you might want to consider a Victron shunt. It's a bit more pricey, but will network with your Victron MPPT solar controller and other Victron devices for better efficiency.
Just to add onto @AnOldUR a shunt can also track and allow you to see how much current (amps) each of your devices in your TAB are using. When you turn on a DC device such as the fridge, Alde, light, you can see the amount of current flowing or amps being used. So it allows you to manage the capacity of your battery when boondocking and estimate when you might run out of battery as you use a device. You also can see when current flows into the battery from solar or chargers.
One reason would be if you have a battery monitor that monitors current through the negative wire via shunt between the battery negative terminal and the negative bus bar.
@ReenieG you may want to hold off on that. The stock factory “wallpaper” is part of the Azdel walls that covers your unit. Here’s a pic of Azdel and what it is. That’s another good reason to never use screws or nails in the Azdel to hang things.
@ReenieG I think this is the one you are referring to, it is great! I can only give you a small part of the address as I downloaded it over 1 year ago. I do not know how to download it to you, advanced age is in the way, it was FOR SALE 2017 TAB 320S FEBRUARY 1, 2022 Hope you can find it Art
We have a 400 and my wife simply used peel and stick wallpaper over the factory wall covering. She installed in on the front wall by the dinette. She got it from Spoonflower and it has been holding up perfectly for the past two years. We've also used in in a couple of rooms in our house and it also has been great. Practically unlimited patterns to pick from. I guess depending on how bad the bubbling on your walls is may affect how it holds and looks.
Battery box on my TAB measures 7.375" (w) x 25.25" (l) x 11.375" (h).
A LiTime 230Ah battery fits in with room to spare. IMO it is easier to have one battery with its own battery monitoring system than paralleling 2 smaller batteries where you may need to worry about cabling, balancing, etc.
@ReenieG, the unfortunate quandary you are in is a large part what prompted this head-spinning discussion now dating back years.
I think the DIY options are as simple as they are going to get. If those aren't an option for you, I agree with Denny16 and Sharon's suggestions.
As a final alternative, if you have a trusted mechanic or handyman that you are willing to pay for their time, they could probably peruse this discussion and fashion the simple tools needed to make the exchange. That's essentially what @MadCityJack did and posted about back on page 11.