While Dale is technically correct, and his method is spelled out in the Prevost manuals find a smooth level concrete surface and just measure from the ground up to the bottom of the bay doors all the way around.

Dales method eliminates the variable caused by uneven ground surfaces, but as long as the space between the bottom of the bay doors and the ground is reasonably consistent it is probable your height is correct, especially if the bottom of the fender skirts is even with or just above the top of the tires.

No sense getting under the coach unless you really want to, in which case, make sure it is blocked. Dale is correct about looking for indications the shocks have failed, but remeber, you should have at least two in the front, and at least six in the rear, with the front two drive axle shocks difficult to see because they are in front of the drive axle front fender liner (I think).