Greg,
The plastic radiator surge tank on your coach is off of a Ford medium duty truck, and it's high failure rate has been addressed many times here on the forum. The only way that I have found to make one last more than a few years is to replace the original 13 PSI radiator cap with a 7 PSI cap, and I would recommend that you do that if you find a plastic tank as a replacement. However, I think that Ford has discontinued the plastic tank, so you may need to replace it with one of the metal tanks that are available from several sources.
Gerald