Lift occurs because of the Bernoulli Effect. It happens whenever one fluid goes faster than another when separated by an object. The air going over an RV would go faster than the air going under, thus creating lift of the RV. This happens at any speed. The turbulence at the end of the RV disrupts the air flow, causing it to slow down. It also keeps the RV from flying.??. So, there will always be some lift at any speed, but I doubt if it would be very measurable and the effect would be negligible. Sounds like the Henley guy has some idea of lift, but not much practical knowledge.
