I read through the whole thread, and Im getting the impression the tank has drained at some point in this process.... at least once. If the dealer drained it, and you used it again, and it now won't drain (again), there's something missing. When the dealer said it drains fine, was it actually empty when you picked it back up? And now you've filled it up/used it again, and it won't drain?
Maybe too much TP and not enough water clogging your system? Somehow over the time period between the start of this post, and today, the system has to have drained.
Mike