Water tank repair
Repairing a polyethylene water tank is very easy and long lasting. The 1976 Palm Beach that I acquired had multiple fractures around the water sensor flange on the side of the tank and had been repaired multiple times using silicon and resins to no avail. Although the GMC parts manual indicated that the tank was a roto-molded polystyrene tank I applied the same technique as I would a polyethylene tank.
I removed the tank and stood it up so that I could work on the cracks. I removed the dead water level sensor and discarded it as I was going to use an external sensor. I scraped away all previous repairs and drilled small diameter holes at the ends of each crack. Because I was replacing the internal sensor with an external I decided to patch the sensor portal. I used a polyethylene cutting board of suitable size that I bought for around 2 dollars and cut it to cover the portal and some of the major cracks.
Using a high-temperature hot-melt and glue gun I temporarily tacked down the patch. Then working with the glue gun and small nozzle gas-soldering torch the type you use for plumbing. I gently melted the surface edges of patch and tank while applying hot-melt to fill the void. Every now and then you have to pause so that the heat buildup doesn't overwhelm the area and you lose a large area of tank. The cracks are done the same way carefully melting to about a third of the thickness of the plastic while working in hot-melt from the glue gun.
Allow your work to cool, fill the tank with water and check for leaks. If all is well, empty the tank and rinse a couple of more times to remove any threads of glue that might interfere with the water pump check valves.