I've had a number of trailers (and a motorhome) over the years. There are only a couple of things on your list that are really necessary.
The most signficant part of winterizing is to make sure that no water freezes anywhere in the plumbing. Your Boler
has a pretty simple system. Draining the city water line, and leaving the faucet should pretty well do it, as long as you don't have a low point in your water "in" line that wouldn't drain. If there's any question, buy an air compressor fitting (schrader valve) for your water inlet, and blow it out with compressed air. If you have a P trap in your sink, pouring a cup of RV anti-freeze down the drain is much easier than having to disassemble it.
Those of us with tanks, pumps, water heaters, showers, etc. have a little more complex chore.
I don't see any reason to lift the trailer or reduce the pressure in the tires
. UV light
will kill the sidewalls long before anything winter storage, pressurized or unpressurized can do to them. I agree that stressing the frame in the middle by jacking up the ends probably isn't a good thing to do. If you MUST jack it up, just use a single jack stand on either side near the axle
mount. The frame was designed to take stress there from the axle
. Covering the tires to protect them from UV is much more important than airing them down and lifting them up.
While it has been suggested that taking stress off the torsion axle
for the winter will make the axle last longer, experience shows that for a trailer with a properly sized and rated axle, there's very little difference that it makes. Torsion axles are designed with about a fifteen year lifespan. Most will make it twenty, and some have gone as long as thirty years before they need replacement!
I have heard that early Bolers particularly are subject to snow weight
stress, so it can't hurt to support the ceiling. "Winter" means lots of things to lots of people, but around here, it's so cold and dry that there aren't moisture issues anywhere. I don't think that "bagging" your cushions is a good idea though, as it can lead to mold and mildew if the fabric and foam can't 'breathe' with the changes in ambient air pressure and humidity. If you're concerned about them getting dusty from sitting, perhaps merely covering them on the top with old towels or sheets would be more appropriate.
I don't know what the purpose of putting a ground cloth under the trailer would be. I'd think it would merely make it more difficult for the snow under the trailer to melt away in the spring.
Good luck, and happy winterizing!