I have experienced an oder from the water in the HW tank if I let it stand for a period of time (week plus) after using it, so I tend to drain it because I don't use my HW every time I go camping. My water source is usually a well system.
I don't use my fresh water tank for drinking water (take drinking water in separate jugs), so I haven't had any issues with leaving some water in the tank for longer periods of time. If it's convenient I'll drain and put some fresh water in.
But I agree with Chuck, I also sanitize my system once a year every spring (twice a year is recommended), it also cleans out the P-traps as well (they can hold odors at times as well).
2016 GM 2500HD 6.0L/4.10
2018 Jay Flight 24RBS
2002 GM 2500HD 6.0L/4:10 (retired)
2005 Jayco Eagle 278FBS (retired)
1999 Jayco Eagle 246FB (retired)
Reese HP Dual Cam (Strait-Line)