Battery compartment space, & battery additives.
I had to replace and relocate my #27 battery from the rear cargo hatch, to the curbside compartment, for more room. I'll have to post pictures, showing how I can fit (3) #24's in the side, behind the wheel well. Had to frame-up a complete compartment with hatch and venting. There just wasn't enough room in the rear cargo area for what I needed. I'm running (2) 2,000 watt inverters for my coffee shop, and need all the battery capacity I can get. (Pics to follow. It's a nice setup. I'll put it under the Registry "aerojava".)
Checking the electrolyte level from the top will be a challenge!
As for battery "additives". I used to work for a CarQuest shop. The owner had many years of experience, and didn't like the stuff. Said it'll gum up the battery plates within a year or two, and doesn't extend the life any.
His point was, if it worked, ALL the battery manufacturers would be using it. None do, and today's industry is struggling to improve battery performance.
I think it's like one of the "Hydrogen Seperators" that promise to give you 10% better gas mileage.
I wouldn't put any contamination into my electrolyte, myself.