Being you have a 2003 which doesn't help much I can only give you some guesses.
1. I would look under your rig and try to find any hoses hanging down with or without any caps.
This is usually where the water tank drain is as well as low points for draining everything for winter storage.
2. If you find the water lines try to figure out what compartment the are below and open that compartment and look for a valve.
Many times the compartment is not easily visible but usually is close to the filter and water fill on the outside.
That is the best I can tell you with little information.
No longer a member here.
Dec 12, 2020