Thanks for all the good info! I have a couple of names of potential suppliers in my area so I am going to make some calls in the next couple of days to see what I can get. Based on everyones comments I will go with the 8mm. I was thinking that I would overlap the sections by about 2 inches then caulk them with a black roofing or flashing type of sealant at those seamsbut I will look for some type of "H" trim to connect them instead.
Don-I am doing this because I have to take a look at my freshwater tank. When I have more than 1/2 tank full and park on an incline where the rear of the 5er is lower than the nose I get water dripping out from under the camper. I had this "fixed" once when I took it to the factory and the problem then was that fresh water fill tube was loose. I don't know if it has come loose again or what is happening. I also want to do the mod where you change the freshwater pick up tube so you can get the all the water from the tank and I might put in different tank sensors (sea-level I think are the ones). That and I just love working on my camper!