Having used several OBD2 scanning tools myself, keep in mind that these readings are not true Engine Dynamometers
that mechanically measure BHP output.
They approximate output based on electrical sensors such as MAF, RPM, fuel flow etc. and through algorithms they give a snapshot of the profile that has been entered. Did the software you're using have you create a profile using accurate perimeters of that engine, & drivetrain, vehicle weight, etc.?
Also note that max HP ratings are generally measured at or near max RPM of said engine at the crankshaft. By contrast the peak torque (which is ideally what you're wanting to utilize with your ISC) is a much lower RPM. Were you pulling a grade at/near max. RPM?
In reality there are probably a number of reasons your seasoned engine might not appear to be producing it's once rated HP, some of which can be pure engineering theory.
The scan tools are often most useful in "trending" parameters and fault codes than their absolute estimated values.
Just my spin, enjoy your coach.
J & J, DRV Suites ES-38RSSA #9679
GM Denali, 3500HD-Max, 4x CC, 8' DRW,
EZGo-TXT, Clubcar Precedent