I just removed the covering from the underbelly of my 07 cruiser and was checking out the the fresh water tank so I will share what I know about mine and maybe it will help you. All of this applies to the tank on mine and may be different from yours. The fill tube attaches at the top of the tank. It is on the top of the tank basically above where the drain tube is located. It is near the edge of the tank and if you open the underbelly you can reach up around the side and access the fill tube. You may be able to access it from above depending on your set up. On mine the water pump is located in a storage compartment and the city water, fill tube and vent line all run down a hole cut into the subfloor and then down to the tank. If I had a skinnier arm and an extra elbow I think I could reach it from there!
The tube slips over a nipple on the tank and is help in place with a hose clamp. As far as the antenna, I caulk around the top of the boot where the cable enters it and have not had a problem. I hope this helps.
Lonnie
Edited by: Chuck N. Wined