Rich, here is a link to a holding tank manufacturer in Elkart, In. They claim to be the number 1 supplier in the RV industry. It may be worth a call to find out if they manufactured yours. I don't know if they could help but it couldn't hurt. Good luck. I went through a problem with my hydraulic lines and was not satisfied until all were replaced by a trailer repair company.
http://www.epi-roto.com/plastic-holding-tanks/