I run the Silverleaf VMSpc program. It also gives HP and Torque readings. They're a guesstimate, but I have to believe that electronics now days can get pretty close. The Silverleaf program takes some fine tuning to get the fuel mileage and mpg information accurate, thus making the other readings more accurate.
On two occasions pulling grades in Oregon, I saw the Silverleaf show that I hit 400 HP and 1186 torque momentarily. My coach is rated at 400 HP and 1200 pounds torque. Since these are engine readings and not rear wheel, I suspect they're fairly accurate.
Side Note: I recently added the DIGI-CRTV computer chip and have only driven the coach three times on short jaunts. I got on it a couple of times and the HP went to 403-406 easily. It isn't the promised HP from the chip, but I was surprised t see it jump over 400HP. We'll take it out tomorrow for the Memorial weekend and have one large hill climb. I'm anxious to see what it does.