I had the exact same issue with my 95 32H Bounder. The riser serves no other purpose other than to elevate the commode. The riser is OEM. All of the black tank is under the floor.
I removed the commode, removed the nasty carpeting on the riser, removed the flange, took off the wood cover and shortened the supports. The 3" ABS plumbing will slide out of the rubber bushing in the black tank. Shorten or replace the ABS as necessary. Adjust length of the freshwater supply. Reassemble and replace the old carpet with vinyl. Add trim as necessary.
I suggest you adjust the new commode height just high enough to allow your feet to be flat on the floor or measure the commode in your sticks and bricks.
I would be glad to talk your through the process, if you like.
Richard
