As several posters have said, the roof truss system is designed to spread the load on the roof pushing DOWN. It is not designed to support a load PULLING down. You may get away with this idea in a stick built house where the ceiling is constructed with 2x4's or 2x6's and designed to sit in one place forever. You're talking about doing this in a structure that is designed to be able to be moved/driven/pulled around on wheels. It necessarily has to be built of lighter, weaker materials to keep weight down. I think you are going to create some major problems which may end up with someone getting seriously injured. Can I suggest that you talk to someone who is an actual structural engineer and not the guy at the lumber yard that sells you lumber or is a welder. If you're sick, you talk to a doctor or specialist not the kid behind the counter selling you aspirin pills.
