Agreed, there is a lot to be said for winter camping
But most of the important things have been covered.
Engine might appreciate an oil change before the layup.
Check antifreeze concentrations
Fill windscreen washer reservoir with winter solution
Chock the wheels and leave handbrake off.
Pump tyres up to maximum sidewall pressure
Batteries - keep charged up or remove
Antifreeze in drains
All pipes, HWS and tanks drained, all taps left on (special attention to cassette toilet and shower rose if fitted.)
(I haven't figured out whether it is best to leave ceramic disc taps open or closed because I have had them stick in either position)
Moisture removing crystals - work better if all vents are blocked up. either arrange for moisture collected to go down thge drains, or check and replace as the containers may overflow.
Allow the pump to run until all the spluttering stops. If pump is easy to get at then disconnect it and run some more. Make sure inlet strainer has no water it it. Water filters - empty and cartridges removed
Fridge doors propped open
All electrics turned off and unplugged
Consider laying lots of rat bait all around the barn to keep the local population down. I put baits inside as well on the basis that I would rather have a dead mouse or two stinking the place out temporarily than a cupboard full of clothes or sofa turned into a cosy nest for the winter.