How To Replace Sewer Gate Valve in Basement?

When I rebuilt the S&S I removed the back 2 feet of the basement floor and replaced it. You might be able to remove just the rear of the floor and fix the valves. You can see the valves and part of the black tank on the right in the pic, nothing moved when I removed the rear 2 feet of the floor. Yours may be similar.

We had to cut the bottom to fix a valve. Rv shops sell the special wide repair tape

My basement/bottom is not removable. The tanks (all of them) are just resting on the basement floor. There would be no way to just unscrew it without all three tanks and god only knows what else would come down with the basement floor. My floor is attached from all four sides by long screws through the aluminum frame and into a 3/4" piece of plywood which was bonded to another piece of 1/4" plywood only slightly bigger around all four sides. Like a board on top of a board with a 1" gap all around which allows it to fit up into the aluminum frame. I'm going to go with a Valterra T-58 and use both dump valves. My original leaks liquid slightly, which will be stopped by the T-58. Very poor design by a now defunct company, not making a way to easily repair the gate assembly. The unit/rig was build literally from the bottom up with no trap door built in for access to repair.

The S&S is the same way, I would have to pull the bottom of the basement off to get to them.

Does the bottom of the Alpenlite basement not come off?

I can get to all the drain valves on our TC, but when there is an issue, the trim around the bottom comes off and then a few more screws allow the bottom plywood to be removed. Allows me to get to any part of it.