$4000 in today's market is the price of a project that needs a lot of work. You will see Casitas that old going for $10,000 in pristine condition.
Your ability to carefully assess all aspects of the trailer, and do you have the time/tools/aptitude/covered work area to take this on?
It all starts with soft spots in the floor and inside lower cabinets. Open outside hatches and check there too.
Casita' bathtub design tends to hold water in and this water will rot the wood in the floor. This water comes from
leaks. No
leaks, no problem! In addition,
Casita improved the floor design over the years. 1989 is prior to these improvements. Still that does not mean the floor is rotted and people repair rotten floors. Google repairing
Casita floor rot
fiberglass rv forum for some insight.
Newbies often think these molded
fiberglass trailers are immune to wood rot. They are not. You just don't have the rot in walls and ceilings (OK, a few designs have that too).