When you say the oil pressure drops going up hills... exactly what are you seeing?
Does the pressure drop immediately when the engine downshifts and goes to a higher rpm? This might indicate that there is an issue with the oil pressure regulator? It could also just be an issue with a corroded connector or bad wire somewhere on the pressure sensor(s)... or maybe the pressure sensor itself.
Or...
Does the pressure drop slowly as the engine is loaded and begins to rise in temp? On long hard pulls this would be normal as the engine oil thins with the added heat. If the oil cooler is plugged or partially plugged, you would see this to a much greater degree as the oil would heat up much faster. The turbo housing is seeing 1200° exhaust temps under load, and the oil is the only thing keeping it cool... and the only thing keeping the oil cool is the heat exchanger between the oil and coolant.
If the engine is only slightly used... I'd say this is more than likely a sensor/electrical issue. That's where I'd start looking anyways, but your not talking to a professional. Just a fellow motor-homer.
-cheers