What rpm was your engine running at? If you're up for playing I'd be interested in seeing how long it takes for the ram to slow from 100 kph to 80 kph in both 3rd and 4th gear on level ground. We would need to know the weight of the truck and or truck trailer combination but it shouldn't make any difference to the reverse hp number that we calculate whether you're towing or not. It would be interesting to see what the braking hp gauge reads to what we can calculate.
Edit: and if you're going to run the test, see how long it takes on level ground to slow from 100 kph to 80 on level ground while in neutral as well.