Hello and welcome. Looks like you have a good project on the go and with the smoke and mosquitoes that we are having a good time to work on it.
A few things:
1- Battery location. Your BF is very similar to my Northern Lite in layout. I also did not have a battery or a way to charge it so I put in a 3 stage charging system and put a battery at the back below the dump hose storage.

I wanted a second battery and there was no place to put one in the TCer so I ran a heavy line to my trailer and added another battery there. In the winter when I do not haul the trailer I put the battery on a luggage rack that mounts into the receiver. I use heavy forklift plugs to connect and disconnect.

2- Heat. Your furnace will not have a problem heating the unit once you get it insulated (consider adding storm windows as well
Link). The one thing that I would like to do, but I do not think that it can be done, is to lower the position of the furnace! If you measure the heat in my TCer in the winter you will find that up in the cab over it will be 28C, down by the counter 20C, on the floor 10C. It only makes sense that a furnace that is not ducted should be lower down to warm up the air/ material around the floor. I think if I did a tear down like you are doing I would add a Cat heater (vented one) as close to the floor as possible to heat the floor / bottom of the camper.
If you are down this way when you get your unit done give me a call!