If the washer is portable just make sure the washer, hoses, fsc and lances etc are dranied down and not left outdoors or in your van (unless it's heated) so frost don't get to the kit.
If you have a garage/lockup store them in that and a simple solution is throw old winter tog rated double duvets over them and rap um up nice and warm.
I once left a telescopic pressure washer lance in my back garden over winter. Needless to say the trigger was fooked due to frost damage...Oops!