Hi qe2 the main thing is to drain the water from the boiler, as if it freezes it will split the tank, unless its a very old RV all the pipework will be in plastic so draining down is not so important.
are any of your tanks exposed to the weather? or are they all inside lockers or in the double floor? in my brave they are all inside lockers or the double floor so unless we have a very severe winter i should not have any problems.
The other option which i favour is to use it over the winter, mainly at weekends.