I have to disagree with some of what your engineer friend said. We have added a sub frame to our trailer and it's nice a leveled now. We took her on a 700-mile round trip a few weeks back and had no issues whatsoever. The trailer did not sway and the center of gravity did not change! Our gas mileage was actually better than before we had her raised. And, the notion that more air would go under the trailer more than the top is silly at best. The air has to hit the TV first, then the trailer. So, our TV's center of gravity has completely changed too? LOL!
Maybe instead of talking to an engineer who (I'm assuming) never towed a trailer, you should speak to a real RV frame shop that has done this mod for a variety of frames, thousand times over? Adding a sub frame is very common for Class A RVs and fifth wheels, and I for one, have not had one iota of an issue with it. Now, I have the peace of mind that both my axles and all four tires will be sharing the load equally, with less risk of tire blowouts and over stressed rear axle!
Originally Posted by The Logans
We looked into it when we got our 25RKS, and talked to an engineer... found out the following: It would void the Jayco warranty, it would void the axle and running gear extended warranties, the center of gravity would be raised, and make the TT dangerously unstable, and decrease the gas mileage (he said because it would be forcing more air under the TT, rather than over it).
All in all, not worth it.
Consult with an engineer... Oh, almost forgot, the insurance company said that they could no longer insure it as it would become a "structurally modified" unit, and it would have to be re-certified as safe.