Reference the mattress, why don't you just remove the mattress from the back of the couch and take it to the dealer. That way they can just do a one for one swap with the supplier and when it comes in, you can just get the new one. This saves pulling the rig in as well. As for the black tank leak. Where is the leak? Is it leaking when you take the cap off to put your hose on? Reason I'm asking is because even after you shut off the gray and black lines, there is space between the end of that pipe and the cap. What I do is add an additional shut off valve on the end where the normal sewer hose attaches. This way I control any gray or black matter that may be in the pipe after I shut the valve off. The major benefit to this additional shut off valve is it brings the whole valve closer to the side of the rig and you won't have to reach as far under the rig to attach your hose. Here is an example of the valve I tried to describe:
http://www.tweetys.com/twistonwastevalve3.aspx
Tim