For possible bypass valve locations, read here:
http://www.forestriverforums.com/for...st-103381.html
If you have a Suburban tank type water heater, then these threads may help:
http://www.forestriverforums.com/for...ore-36197.html
http://www.forestriverforums.com/for...es-135977.html
Assuming when you say drain the water tank, you are meaning the water heater tank:
The Suburban brand uses the anode rod as the drain plug as per the pic below:
If you have an Atwood brand tank type water heater, then the drain plug is a nylon/plastic plug, as per this pic below"