Adding a bath to the smaller trailers eliminates 90% of the choices and will increase price. The Scamp
13 has a bathroom model. Casita
made a 13 foot model with a bathroom too. Hard to find and they stopped making them several years ago.
Since Casitas are made in TX, maybe you will find a Casita
13. If you see one, pounce!
In the 13 foot size, the trailer body is only 10 feet long. To get a bathroom you have to eliminate something else. That something else is a place to sit separate from the bed.
As far as breaking the bank, molded FG trailers are enjoying record demand and pricing is up, up, up!! In the world of RVs, molded FG is at the expensive end compared to overall size. On the other hand, they don’t depreciate like most RVs. People are selling five to ten year old units for what they paid for them!
The other thing in TX to consider is AC. Most vintage trailers are not set up to accommodate AC. A newish Scamps will be built to properly support roof air. The Casita
13 was built to have a small room AC in the front curb side corner.