Always do mine when I think it might have had its last outing of the year and if it's not then it doesn't take to long to refill the tank and flush out the pipes and taps etc. That way if we suddenly get a cold spell I don't get caught out, hopefully.
The Miller had an auto dump valve for the boiler.. I reet pain in the nethers it was too!
Fine if you are parking the van up over winter, but the only way to keep it emptying the system when travelling around mid winter was to cable tie it closed any way !
The Autotrail has got a manual valve, BUT it also has thermostatically controlled taps on both the shower and the external shower
When we bought it we found that the previous owner had not done a proper drain down and it had ruined the internals of both
Managed to get a replacement shower mixer unit ( £84 retail !!! ,,,, Not that I pad quite that much ) but have yet to source a unit for the outside shower.. No biggy as I will only ever use it for cold water delivery nyway