I pulled the underbelly on my Jayco a few times. Look carefully at the screw heads, they are probably Robertson (square drive the looks like a Philips) I bought a cheap set of drivers at Walmart that included the #2 Robertson bit that fit in my cordless.
Just take off anything that is holding the underbelly on, in my case it was the two rear stabilizer jacks and a strap under the drain pipe. Then Jayco also ran the propane line for the water heater right up through a hole in the underbelly. I used a razor knife to slice a single cut from the LP line to the edge of the underbelly. Then just lay on your back and take out all the screws around the underbelly near where you need to work. In my case Jayco used a smaller piece 5x5 under the tank so i didn't have to drop the whole underbelly down.
For reinstall it helps to use a floor jack and small piece of wood to lift the underbelly up while you install the screws. It's not that the underbelly is particularly heavy, it just helps take the sag out of the center so you can line up the screw holes easier.
Oh and a new valve is $14 which is way cheaper than the gas to pull the trailer 100 miles.
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2001 Suburban 4x4. 6.0L, 4.10 3/4 ton **** 2005 Jayco Jay Flight 27BH **** 1986 Coleman Columbia Popup