I just had the same problem on a 2000 Beaver Patriot Thunder, the tanks are from an F-85 Series heavy duty truck, its not made anymore, finding one at a truck repair shop is like hunting for a needle in a haystack, and for the plastic ones they are asking between $150 to $200, the one that was in my coach was replaced 2 years ago for $150 plus installation. I called Beaver Customer Service and explained my problem and they stated that they have made one out of all steel construction with a sight glass built in, and a lifetime warranty but they cost $400. I bit the bullet and ordered the steel tank and did the install myself, 4-bolts, 4-hose clamps. I'll never have to worry about it again. I dont know if your tank is the same as mine, but I wouldnt be surprised if it was.
Jeff & Marcia .... RVM 39
2000 Beaver Patriot 425 Thunder 40'
2008 Jeep Wrangler X