Michele, If this is your RV, the water heater is behind the outside access panel, I have the red arrow pointing to.
To drain it, if it's a Suburban brand, you remove the anode rod at the bottom front of the unit. It requires a 1 and 1/16th inch socket to remove it. I included the link below that also has a chart so you can inspect the anode rod while you have it removed, draining the tank. You have to replace these rods as needed on the Suburban brand heaters, and is a regular maintenance item.
Drain the hot water heater?
If it's an Atwood brand, there is a drain plug (plastic) on the bottom front.