First, hopefully the grey, black and fresh water tanks are all empty or very close. I would pour anti-freeze into each drain in case there's still water in there as well as if they dried out the odor won't come up through the piping. Also pour some in each toilet to protect the seals of the ballvalve. Check the water filter canister and remove the filter and any water. The water heater needs to be drained. If there a way to get a small compressor there with electric she could blow out the lines. If not leave all the faucets open, with the small amount of water left, it shouldn't present a problem.
Where is the trailer located? There might be someone close by on the forum that could help with the winterizing.
Be safe and thanks for your service from us all.