Spoke again to Ford. They say the test for the parking brake is to park on a flat area, put the motor home in drive and do not touch the accelerator. If it doesn't move then it performs to design. I agree with edprice, I wish I could find a published standard for the capacity of the parking brake. I do not live in a state that requires an annual safety inspection, I wonder what these states use for a standard to pass a parking brake at inspection time? At my next opportunity, I will apply the Ford test and see what happens.
No matter what, chocking the rear wheels is a good safety practice.