If I had it to do over again...
1) I would have bought one of the first trailers I looked at, rather than spending 2 years searching for "the one." Had I done that, I would be finished with my renovations by now! I didn't realize at the time that any 40+ yr old trailer would likely be a shell-off.
1.5) I would have resolved not to drive 600 miles to buy the trailer (it is just dangerous, as you don't want to go home empty handed).
2) As soon as I realized I had rotting floors and rear-end separation, I would have built the gantries and pulled off the shell. I would have then used the gantries to flip the frame over, and would have taken the bellypan off with the frame upside down. This would have saved me a few hours on my back with crap falling in my face.
3) I would resolve from the start not to be such a cheap bastid. For example, as the reno began, I was on the constant prowl for used RV parts like a replacement fridge. Now that I have bought so many new things, the used junk that I bought early on looks old and tired in comparison. Should have just resolved to spend the money in the first place.
4) I would buy a new house with a massive air conditioned workshop on site, so that I wouldn't have to suffer from the heat, cold, mosquitoes, blazing sun, and poor lighting. Man...this renovation is getting expensive...
5) Had I known the troubles and costs associated with replacing a vintage formed segment with a modern alloy one, I would have been a lot more careful with the original part!
6) I would not have so willingly gone the route of replacing all of my interior skins with fresh aluminum. This was expensive and time consuming, and has so far only delayed completion of the project. I bet it would have looked fine painted. Besides, by the time all of the cabinetry, bulkheads, drapes, cat pictures, etc., are hung, you won't be able to see the darned walls anyway.
I am just about to start working on the actual rebuilding of the interior--I'll update this post once I have a list of regrets from that experience as well!