I have a 2015 Silverado 2500 4x4 with a 6.0. When towing my camper it will not upshift out of 4th gear under 70 mph and runs over 3k rpms. I can't force it to upshift in manual mode either. I would like to be able to have complete control over what gear it is in while in manual mode and force it to upshift into 5th and 6th gear so I can run at lower rpms at any speed. Can this be done?