I also left all my cats in the netherlands outside in wintertime. (Though just this summer I built a open shed around it).

There are three issues which I always carefully check (besides the possible water inside the hulls):

Each cat has a depressurize inlet/outlet and drainplugs. When you let the drainplugs out, take care for possible waterinlet. When you leave them in, be sure that the depressurizing works.

Also make sure that the beams are level to each other on the trailer (beams in each others horizon), so there is no torsion in the connection points. Support the girders for the hulls on the trailer if necessary seperately.

Be sure that the lying mast has no waterinlet.

succes

ronald