Hi Don! Welcome to the Forums!
I split your post into a new thread as I think it is significant, and deserves to stand alone.
In response, usually transmission coolers are required on automatic transmissions in response to heat build up caused by plate/band slippage (or some such thing... automatic transmissions work by magic, as far as I actually understand them). Manual transmissions are just a gear box and don't require any additional cooling (generally speaking). I'd expect your Pathfinder's tranny will do just fine. The heat buildup and expendable parts on a manual transmission are the clutch plates.
I'd suggest that the appropriately-sized weight
distributing hitch would be in order. I'd also recommend that you use some sort of sway control; whatever is appropriate for the tongue weight of the Casita
Best of luck!