Funny somthing linked me yesterday to the thread where you swapped in your current battery. As to the battery-most myself and you would reccomend replacing both batteries. Though optimum I dont believe mandatory. Suppose it depends of battery condition, at what point or age it becomes deteremental mixing I dont know.
When I added our second battery, a gr29, original gr24 was almost a year old-used them for 5 years. The differnt batteries were tied to a perko switch. Intent was using them seperatly but within a year I found longer run times when using both simutaneuosly-even though differnt size- even worse-second battery was at end of 15' 4ga cable.
But it worked and was a gain.
When we added solar I replaced batteries and rewired in true parralle. I can without hesitation say the differnace was huge. Im sure having 2 like gr29s was part- but rewiring, the gain was MORE than adding the second battery origianlly. In power availalbe-holding , recharge times.
So My number one recommendation is somehow someway to truly gain the advantage of adding a battery is wiring in parralel. Even at the expense of longer wire lengths IMO. Which if one needs to move around not sure how-seems your aware of this.
I have a Torklift hidden power setting here on my bench, was going to add but but requires redoing my air comp & decided not practicle. I was/am going to mod so it would set on drivers side frame rail. Just havent goten round to it. If you were closer Id let you try to mount it. I think too large though for your truck.
Free/loose flooded wet lead acid battery inside of camper-NO.
If it were the only thing between me the zombies clawing at the door-would still be a hard choice, Ive seen some of your projects and am sure your clever enough to figure somthing out. More thought required me thinks.
How much space between camper and truck bed walls or depth of wheel wells? Room and still loadable with box added to front of camper so battery stayed with it?