The water heater is drained by removing the plug at the bottom located behind the outside access door. Removing the pliug will take a 1 1/16" socket. A short extension is also usually helpful. This is the same plug that has the anode rod (unless you have an Atwood heater, which has no anode rod) is attached to. Open a hot water faucet inside to speed draining.
On our '05 Allego Bay, there is a bypass valve in the wet bay. Just flip the handle to the bypass position.
If you don't have a bypass valve in the wet bay (or elsewhere), there will probably be 3 valves inside by the water heater. Close the 2 that go to the inlet and outlet, and open the one that is in the crossover pipe.