...also, take a look through the "sticky" thread on the top of this forum which will give some good information too.
Tow Bar Angle and Height Difference
eta: I believe Roadmaster has revised what is posted toward the end of the above thread and is now recommending 3" up and down.
or see attachment below
Also, I believe Blue Ox recommends never towing at a negative angle (or having the toad higher than the motorhome) and only 4" or less on the positive side (or having the toad 4" or less lower than the motorhome).
So in a nutshell?: Roadmaster- 3" or less positive or negative, Blue Ox- 4" or less positive, and "do not tow" negative.