I don't have a clue what the gpm or pressure should be... . but when your blasting up a nice grade with a trailer... I'm guessing that the close to 1200 * that's coming out of the engine would cook the oil if it wasn't at least 1/2 or 1 gpm... after all, most of what the oil does in this application is take heat away from the bearings so they will live.....
Just my thoughts... .