This was discussed a while back. Here is the link
http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/26287126.cfm
I drained my hot water tank every year, but this spring I used the vinegar procedure to rid the heater of lime scale that was breaking off and clogging the faucets. We just returned from a 2 1/2 month 8,500 mile summer trip and had no trouble with lime. The vinegar procedure WORKS!
'05' F-250 Power Stroke
'00' 30' Cameo Fifth Wheel