Delamination?

This sounds like a complete money pit. Water is a killer to any structure. Even if you fixed it I would still be concerned with mold and structural damage. The amount of money you would spend would be lost because it will never be worth what you put into it. I would estimate the cost of the job (likely well over $10k) and take that money and buy something that is in decent shape with no leaks. Plenty of used Rv's out there.

If the systems all work, and the drive line is good ...part it out on craigslist! Then look for something without leaks or delamination with the additional monies from the parting out.

If the roof is replaced and properly sealed/maintained, it will eliminate the leaks and additional damage.

I don't have any advice for the delam repair. I know it's a major project.

In my opinion, you can spend a lot of time, and money, and even after it is repaired, the 1992 Monterey may still be only worth $1200.
I believe the only way that you can properly repair delamination is to completely remove the side wall, take it apart, replace all damaged wood and insulation foam, apply a better adhesive, and then put it all back together again using compression to hold the sidewall flat while the adhesive sets up.
To me this is not s DIY project and should only be attempted in a factory type environment. Even then, the time and materials involved would NOT increase the value of the coach being repaired. Of course, there is probably interior ceiling panels that also have to be dealt with as well as rotted floors from the leaks.
Stopping a roof leak is pretty simple basic repair work. Dealing with the delamination and other damage that has already taken place is the huge problem.
I would find another project Class A without delamination issues.