1991 western wilderness water tank

Bottom line, joe is right. I don’t know about polypropylene but I have installed or attempted repair on 25-30 polyethylene tanks . Repair is successfully completed thru plastic welding. Harbor freight sells a “pos” tool. Hobie kayaks has a good(read expensive) tool. The problem is that you have to melt the plastic for it to bond, very dangerous because you might melt too much then you end up with a gap. Good to have a spare piece of polyethylene. Repairing modern rotomolded kayaks the same. As far as installing an access port, it will work as long as you don’t over pressurize the tank, such as filling until full. One of the great advantages of polyethylene is nothing sticks to it well, including sealant. You still face the problem of repair if you take this route. When I was in business I ordered tanks from Ronco ( Ron’s company), on the west cost I believe. Back then they made many tanks of different shapes and sizes with no holes. I would call and order a tank and specify hole size and location and they would spin/melt a fitting in where I wanted. Hope this helps.

Tank if I recall is in the step up to the cabover.

Remove the staples/screws that hold the front wall of the tank step in place, carefully separate the paneling from the frame behind it so you can find the anchor points for the frame.

Remove anchors, remove front step/tank compartment wall. Disconnect all plumbing connections to tank (Most should be hose clamps), remove tank.

Replace tank, its polypropylene plastic and its **** near impossible to mend, reverse process to rebuild step, avoid drilling into tank in future.

Try a universal rubber winter plug.

Can you access the top of the tank?
You could install a water tight marine hatch (something like this: https://www.amazon.com/BBH-Marine-Round-Non-Slip-Plates/dp/B09DS3YYV2) on the top and patch the drill hole from inside the tank.