RPMs dropping uphill, Ford f-450

RPM declines with a decline in speed would be normal. WOT would be subject to some fluctuation in speed. After a downshift the torque converter could be unlocked and you may see a drop in RPM as it locks up again.

What is the rpm at and how much does it decline?

Assuming you are around 25,000 lbs total. 100+sq ft of frontal area. You need 150-175 hp to go 60 on a level ground. Another 50-55 hp per 1% grade. You are needing more HP than you have. Yes you should be slowing down some.
Also, I don't believe a 2013 has 400hp. But could be wrong.

Marty

Not trying to be smart Alec here and not saying there may not be something wrong with the vehicle as there may very well be but hard to say from the minimal description of slight noise and vibration

If that didn't happen every vehicle that tows (or otherwise for that matter) would not have a problem going up hill, hauling a load etc. Those dual cooling fans, expelling heat from the engine and or transmission whether through oil, water or air all take their toll on horsepower (dont get me wrong not implying a lot of horsepower but a couple here or there start to add up).

The work (in regards to physics) required to move a large load uphill is significantly more than on the flat. Even though you say the truck should be able to handle much more you still have a very heavy trailer. Dry weight 14500, loaded light perhaps it is only 15500 maybe more. Not to mention the frontal area of the toy hauler not quite as good as a sail but not far from it. That is going to put a lot of load on that engine 400 hp or not.

You might try downshifting to direct drive and see how it goes (not that familiar with fords so unsure which gear that would be on my ram it would be 4th gear). You may not be flying up at 65 or 70 but you may be able to hold speed and rpms. When in top gear in overdrive or even double overdrive that puts additional stress on the engine and transmission for sure when it starts to struggle and will lose speed and rpms.

Diesels are great but they do have their limitations. The f450 power wise is not any more capable than the 250 or 350 assuming the same engine. 400hp is 400 hp it can only do so much.

How much rpm loss is there, corresponding speed loss how much, the noise when does it start, where does it seem to be coming from, the vibration when does it start, where does it seem to be coming from (engine bay, underneath truck, rear of truck etc), what speed and rpms are we talking about?

Are you in tow/haul mode ? Did it downshift ?

What was the speed at the bottom and how far did drop ?