Not sure about your Itasca, but on mine there was a 3 or 4 inch long by 3 inch diameter connector, looked like rubber, between the tank and the pipe. When I purchased it used the owner said he never used the toilet or the shower. The first night out in paved parking lot parking we left a long streak, followed by another in the morning. This plumbing fitting caused a lot of concern and grief until I actually identified the part as a common plumbing fitting used to connect three inch plastic lines together. Once I did that, any plumbing supply store can provide them, and I replaced both on the rig. Total cost was $8 each. About a 20 minute job, a bit messy.
Totally not sure if that is your problem or not, but it's worth checking. Flush everything out where you can and when you can before you begin.
My connectors were so rotten, they were literally falling apart and would have split open totally at any time.