Allison transmission question

[almckin]

I have a 2003 GMC Seria 2500HD and 8.1engine with 373 rear end.I dont have any problems When towing my 5th wheel rv that weighs 10400 lbs from dead stop.It shifts through the gears just fine however when you come to a grade it wont down shift to pull the grade so the engine wont labor.I always tow in tow haul and OD locked out.Could my transmission be malfunctioning with out triggering a check engine light?

 

 

 

Thanks

Al

[ChevyTech007]

I could not find any bulletins that apply. I would take it to the dealer and have it checked for codes and possible updates.