Not sure if you're asking about what to do during winter storage or spring prep, so I've kinda added both.
I've got a tent trailer that I park in the garage.
Mice are known to go through the little holes and wreck havoc on interiors. So, the tent trailer forum guys suggests going around with a flashlight and where one can see light through a hole, fill them up with silicone. I do that at the end of camping season
I have a WFCO 3-stage converter, so I leave the trailer plugged in 24/7. It will float charge the battery, so I don't need to remove it from the trailer and throw a charger on it every couple of months. Lots of people swear by the Battery MINDer, but I've never used one. Others take the battery off and store it in their basement, and charge it every month or so.
I also have it jacked up and resting on blocks (off the main frame). It keeps the pressure off the tires and won't cause flat spots (and excessive pressure on one spot of the trailer bearings).
Come the spring.
I walk around, make sure all the tire pressures are good and they look ok, no dry rot or cracking; repack my bearings; check the 7-way bargman and test the emergency break-away switch (make sure, you're not attached with the 7 way, it's been known to fry brake controllers).
Flush and sanitize my water system and check to see that all appliances work (fridge, stoves, water heater and furnace).
Also check the propane detector, replace the smoke detector battery and turn the fire extinguishers upside down and wack the bottom with a rubber mallet--the powder settles and the bouncing from traveling down the road, seems to compress it, this way, it breaks up the power.