I pulled the plywood top off the box under the mattress and found the drain valve for the fresh water tank. It's a 28" reach from the top of the bed frame to the valve, so I cut a notch in the end of an 18" piece of schedule 40 PVC so I can use it as a wrench to easily reach the drain valve. I also put a 4 foot piano hinge on the plywood top so it will be a lot easier to get to everything under there.
I also called Shurflo tech support and asked them if it was ok to run the water pump long enough to pump the water out of the fresh water tank. They said "yes, it should not damage the pump". However, they did say that if there is anything in the tank that could plug or damage the valves in the pump I could have problems. I'll be using an external water filter from now on.
Blessed are the cracked, for it is they who let in the light