Hey barkley1956, I found this link online from another Pop Up forum. They are discussing the repairing / removing an undercarriage water tank with the lift system in place, just as you described. One poster stated he replaced his and says it was pretty simple. May want to ask them about yours. Thankfully my water tank is onboard, but doesn't make it any less painful if it should ever need replacing. Good luck.
http://rvitch.com/forum/index.php?topic=62754.0