If u leave water in the system its not a case of it might get damaged. It will get damaged.
1 remove carver boilers external white plastic drain plug and place in kitchen sink.
2 open carver external top, fast drain valve.
3 open all taps.
4 If swift group van, & some other oldervans, remove the red bung/bungs called low level drain bungs, & place in sink.(these are located under the van & not necessarily anywhere near the heater.
5 if newish Truma boiler, remove, 5 amp immersion heaters spur fuse, (if it has spur) or ensure switch is off at the consumer unit, Raise the yellow fast drain valve to the vertical.
6 drain & clean any onboard water tanks.
7 empty toilet cassette.
8 empty toilet flushing tank.
9 u could also drop the shower hose to the floor after removing the shower head & leaving it in the sink, this will get any water out of it.
This is the state we leave all our stock vans in and any Pdi'd & sold is given to the customer like this, from the 21st Oct, until may'ish