Hi Doug, Happy to tell you what I've done. Others may know better.
We did a trip to New Brunswick from Ontario last September. I saw no reason for sway bars. May only need if trailer gets "tongue light" because of too much weight in the back. But we took a bike rack and a bike and I had no sway issues - even on 4 lane highway.
For fans, I installed a Caframo Maestro which is speed adjustable . Put it at out feet on the ceiling. Our sailing friend says Caframo are the best fans. I just bought a 3way 12V receptacle splitter on Amazon and plugged that into 12V inside trailer and wired the Caframo fan to a 12V jack (also from Amazon). The lowest speed on this is great for the small space. Highest speed is too much. The interior fan you ordered should be fine. Also, we did some October camping in 2022 at Bon Echo Provincial Park. Some nights down <3degC. Internal condensation was an issue so I just had a MaxAir fan installed to allow us to exhaust air outside while we sleep. Condensation not really an issue on warm nights.
Your 2 batteries will be great and you'll be happy with that. I chose to get the extra AmpHours (Ah) with one larger battery (but less weight than 2 batteries).
Hope that helps.