You say you will never use the fresh water tank but are you new to the seasonal place?
I have a seasonal site in the 1000 islands and always have some water in the tank just in case the water goes out. We are on a well and if the power (hydro) goes out we have no well water. This happens once or twice a summer for no more than an hour or two but it sure is nice having water to flush with. We just chlorinate the water heavily.
If you elect to leave the wheels on make sure they are covered but if you intend to leave it on the site permanently take the wheel off and put them in your basement or garage.
You know of course to keep the drains shut specially the black water tank and open it to dump when it starts to get full or before you leave for an extended period (over half full even if you have to add water).
Also make sure water is off each time you leave for more than a day.
Buy vent cover for the vents specially for the winter. Hate to have one break in the winter and find the damage in the spring.
Do not skimp on antifreeze when you winterize the peace of mind on really cold nights is worth the 5-6 dollars extra it costs
To level the trailer we use cinder blocks on railroad ties to try and distribute the weight enough to keep the unit from sinking too much