You should be OK. If there's no water in the fresh water tank to push out the water in the hot water heater, the water in the HWH will stay there.
The water in the HWH is usually considered to be "unusable" in terms of determining how much water you can carry. For example, FR may tell you that you have a fresh water capacity of 32 gal, but 6 of that is in the HWH, so you really only have 26 gal that are "usable."
1988 Coleman Sequoia - popup (1987-2009) - outlasted 3 Dodge Grand Caravans!
2012 Roo19 - hybrid (2012-2015)
2016 Mini Lite 2503S - tt (2015 - ???)
2011 Traverse LT, 3.6L, FWD
2009 Silverado 1500 Ext Cab, 5.3L, 4x4, 3.73
2016 Silverado 2500HD Dbl Cab, 6.0L 4x4, 4.10