Building Your Own Travel Trailer?

If you enjoy building stuff then go for it. If you want to take a short cut just find a used trailer that does not have water damage and update it with things like led, fantastic fan, High profile foot flush toilet, thick foam bed. The resale will be better too. Home made is knocked down considerably at resale. Yes, I know this first hand. It wasn't cheaper in my case and it was so much fun I wouldn't do it again.

The other issue is acceptance by campgrounds.

Some campgrounds, because of insurance regulations, will not allow home made campers.

People have been repurposing pop-up campers ever since I was a kid (and I'm 60 years old now). My parents neighbor took an old pop-up and stripped it out, put folding sides on it, build a bunch of slide-in photo frames, took the door side and build a folding shelf over the top, and used it as a "Booth" trailer where they sold photos at auto racing parks in Central Indiana. They were the official "track" photographers.

About the legality of rebuilding? As long as the frame retains the VIN number, you can pretty much do anything with what's attached to it you want... provided you are within the weight limits of the license and type of trailer.

In Indiana, Pop-Ups are not considered RV's. They are considered Utility Trailers, same as a utility trailer you might use to transport a riding lawn mower or move mulch in. You can take the frame and build anything on it you want. It's still a "utility trailer".

There are no laws preventing you building your own car, truck, RV, etc.

Any registration rules would be specific to your own state. I suggest you research with the department of motor vehicles in the state you intend to register the vehicle.