Don,
As I said in my previous post, the Shurflo pumps have a clearance problem on some coaches. Some owners that wanted a Shurflo pump regardless have cut out the compartment cover to allow for the Shurflo pump's larger size. The Aquajet or the Flojet variable speed pumps should both fit into your compartment, but measure for clearance before you purchase them.
As for the water flow when the pump is removed, it is from the fresh water tank. The fresh water tank is above the pump on almost all coaches, and it will drain through the supply line to the pump if it is disconnected. If you are not dry camping, just turn off the campground water supply and drain the fresh water tank, and you can take your time to change the pump. If you are dry camping, you have to "hot swap" the pump very quickly to prevent loosing your water supply.
Gerald