I think you're going to need something custom. Are there any yacht dealers or marinas near you? Up here in MN most of the yachts come out of the water in the winter and they sometimes have covers they put over them. I suspect it would be quite the job to put a cover on and off. Where are you going to park it? Is it at all possible to build a roof over it? That would for sure be much easier. Might not even be much, if any, more expensive than a cover.
Congratulations on the new purchase! Looking forward to finding out more about it and seeing the pictures.
'03 Freightliner FL112, 295" wheel base, with '03 United Specialties 26' living quarters, single screw, Cat C12 430 h/p 1650 torque, Eaton 10speed , 3.42 rear axle ratio