Love all this seasonal chatter by the way...
We've been lucky enough to go up every or every other weekend so electric stays on for us. Water gets turned off before we leave every time.
I agree with your treated lumber choice. We leveled ours on 2x8 stacks. The ground isn't sandy but as long as you're foot print is spread out enough you should be ok.
We also lowered the stabilizers onto a small piece of lumber placed on top of cinder blocks. Stack them as high as possible, the less distance between the blocks and the frame, the less bounce and more stable the trailer will be.
We stack cinder blocks topped with treated lumber under the frame in front and back of the axles for additional support as well.
I wouldn't worry about keeping it on your site during the off season. Even in this awful weather. In fact, mine is there right now along with everyone else.
Have to agree with the staircase too. All great ideas from everyone.
Best wishes on your new adventure.