OK, got back from a long weekend trip and looked at my tank today. This in a 2004 Allure, 36', with a 135 gallon tank. Forward wall of forward bay, passenger side, opened the inspection panel, dusted off the top of the fuel tank, unhooked 3 wires and removed 5 screws, and pulled the sensor from the tank. I have 13.5" of clearance above the tank and tank opening of 1 5/8". As you lift the sensor, tip it toward the rear and it comes out the wall opening without any difficulty. Reading the directions on their website, they replace these with a 1/4" tube if less than 24" in length and mine is 17.5" with a 1/2" tube. Will call them tomorrow and order one. Price is $64 plus $9 shipping as near as I can tell.
Don, I bought this coach new in 2005 and my coach, when filled, stays on full for 150 to 200 miles. Then it slowly drops until it gets to half. The bottom half goes down more quickly and I usually refuel at 1/8 to 1/4, depending on where I'm at or going to and fuel availability. Even with these readings, 70 gallons fills me up so I still have 65 gallons in the tank.
I would assume CC calibrates these tubes when they're installed and they may set them a certain way so you don't run them out of fuel. The gauge I removed has calibrating screws on the top, and the Centroid website shows in their install instructions how to recalibrate the sensor if you choose to do so. I probably will leave mine alone as I maintain a log of each trip and I have a good idea of when I need to fill the tank.
RJPerry, are you sure that photo is the only access you have to the tank's top? I'd get under that rig and inspect with a bright light for an access panel you may have missed. Good luck.
Hope this helps.