The short answer is it depends. I don't think you will have any problems with hanging the shell from the ceiling. However, that assumption requires additional investigation. Trusses are designed to a certain capacity depending on the span, the dead load and the live load expected in the building. Dead load is all the items required to build the roof (sheathing, trusses drywall, mechanical systems conduit etc.)in other-wards all things installed at the time of construction. Live load are temporary loads like snow, people, boxes etc. it is a very specific number based on the planned use of the structure. This number is determined by the code used in your area. So any load that is placed on the trusses after construction that was not originally designed for will start eating into the live load. What I have seen happen is that people will start using the attic of their garage and think everything is OK becaouse the roof is not sagging. However, come winter and a large snowstorm and all of the sudden the garage roof is sitting on top of your car. Your camper could be the straw that broke the camel's back. Your post does not contain enough info to say that the roof can hold the shell. Only you can do that by evaluating the structure and how the garage attic and roof. So the closer the existing structure is to the as built condition the more likely it is that you will not have any problems. I am an Architect who worked as a code official for many years.
