I am pulling a 5th wheel and having power issues towing uphill. Took to mechanic and they replaced the screen filter, fuel transfer pump and fuel filter. Issues still happened, took to different mechanic and he said air filter was almost sucked up in the engine and the stand pipe was getting stuck. And after he fixed that, the fuel pressure was where it needed to be, where it was not before. So we towed again and ran good, until got to some big hills and lost some power but was okay, then toward end of 7 hour trip, had issues up large hills and at one point the service engine light came on for about 10 minutes with reduced power and then it went off and ran fine until one last large hill which it could only go 20 mph up.

The mechanic did get these codes but thinks it is due to the chip that was on there. dtc p0603-p1211 & DTC 1298.

Any thoughts??? Thanks!