I use a paper towel that I stick in the socket hole and hang it down the side of the camper. You will be amazed how much water will wick out with a paper siphon. About 10 minutes after it stops wicking it will be bone dry inside.
DO NOT forget to put the anode back in! It will keep the threads from rusting and bugs out of the water heater.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW