Still haven't found the water pump

I have a 2008 21QB as well and the access for the pump is through the outside storage door at the back of the MH. Open the door and as you are looking in the storage area, you should notice a small panel on the left, about 6" x 10" held in place by a couple of screws. This gives you access to the pump, which is, in fact, under the shower. It is a PITA to get your hand in there to work on it. I did a mod where I put a faucet with a shower valve in the bathtub (I hated the shower hose connected to the sink faucet) and I had to cut a hole through the inside of the storage area, above the panel for the water pump, to gain access to under the shower. Nice to have small MH but they have to fit everything in a small space.

Another option might be to remove the water fill panel at the rear, but I would be careful. Good luck!