It's probably an Atwood heater, but that really doesn't matter. There is an outside access door. Just open it up, on the lower left side you should see a big plastic 6 sided nut, which is just a screw in plug. Just unscrew that until it comes out, water will drain.
If you want to drain faster, there is a brass pressure relief valve. It has a metal lever on it at 90 degress. Just lift it up to 180 degress. That will allow air into the top of the tank, draining it faster.
John & Clare - Buck & Zeus - Jack & Husky
2008 Fleetwood Bounder 38P
Towd: 2 HD Streetglides