Using winnebago box
 

I know this is not really a truck conversion but I have a few ??s about taking the house off of a 25 foot minnie winnie e350 and putting it on a f550 crew cab 4wd chassis. Specifically I need to know what is involved with the cab to overhead connection. Anyone ever done anything like this ?

Well it's a truck and you're converting it, right :D
I've heard of people getting an "accordion boot". I believe that there are two thicknesses, the bigger for when the cab is air ride, the smaller for when the cab is fixed.
That's the end of my knowledge but it should give you some search terms.
Sounds like a cool project, don't forget to share some pictures.

The first problem your going to have is the truck frame. The Winnie is based on a van cutaway chassis and is flat all the way back. The truck chassis is curved up over the axle (usually) That will cause a whole world of problems. The second issue is going to be axle location and tank placement. Measure all of these first and I can give you some pointers from there.

You sure about that?
I'm 87% sure the F550 is a flat MD frame.

Check to make sure your 550 doesn't drop down under the cab, then come back up for the flat frame. All the ones I have seen do, although I'm not by any means an expert on f550s. There are going to be year differences and such. I would hate for you to get to the point of re-attaching things to find out they are different.

Do it!
 

I've always thought about doing this but the problem I run into is making the box (think earth roamer fiberglass box). Your idea is actually a world better than mine because the box is already furnished and all the specifics are already there - all is needed is some re-doing and modification.

I'm in the process of stretching my f350 crew cab to have a 12ft bed, dump bed, and a box... All of which, would be able to slide on and off. Wouldn't mind a winnebago box mated up to my truck.

The box to truck connection is tricky. I dreamed up an idea on how to do it. Can't really show you b/c I don't have it drawn down but picture this: Your box will have a donut cushion on it, the truck will have the female donut shape. Once you slide on the box, it will be a perfect seal. I use to have a photo of it from another truck and it's similar to that.

I have a 4x4 converted mini Winnie. IMO the box is the weak point, its just not designed for any rough road type environments.

If you do end up doing this, I would look for the two piece fiberglass type campers, Bigfoot etc, to use. Their shells are much more robust.