As you mention 7 degrees, I assume you have one of the newer blue/grey frost control valves and a Truma Combi.
These are designed to dump water from the
combi boiler under frost conditions, though a by-product of this
might well be to drain the cold water tank, depending on water system design, especially if the pump is switched on.
The valve is designed to dump at around 3 degrees, and can't be reset until it is above 7 degrees.
If it's mounted near to the boiler, and the heating/water heating is on, then this is usually sufficient in UK conditions to overcome any risk of dumping by maintaining an ambient temperature of above 3 degrees at the valve. (not guaranteed though if it is really cold). When away from the 'van I would use space heating only (no water heating) just in case the water does dump.
The older series of (red) valves were electrically operated (rather than the current "passive" operation). As such, the valve wouldn't dump if the heater was switched on, being held by electric current. If you are concerned about dumping, and have enough occasions when you think it might, you could consider retro-fitting the Truma accessory that effectively replicates this on the newer valves (it also assists with getting the valve up to temperature to refill). It applies heat to the valve when the combi is in heating mode.
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