I started out years ago towing our 7,000 lb trailer with my old chevy 2500 with the 350. I tow mostly in the Sierra and Rocky Mts. I hated the way I had to slow down to 45 mph going up the grades. So ...
It depends on the grade. You have gained 10 MPH and I think that's about right. My C with an E450 V-10 has encountered some grades where it drops down to 3rd gear (I think) and rev's up to about 4K or a bit more to hold 55.
And I'm pretty sure that all of the V-10 truck engines are not the same. Seems like I remember that there are at least two distinct versions.....with different displacement and horsepower.