How steep a hill was it? If the tach was only showing 2500 at 45-50 mph then I assume you were in 4th gear. To accelerate a car as heavy as a 420SEL going up a steep hill you'd have to get into the higher RPMs. Get a second opinion on this as I am no expert, but it sounds like it just didn't downshift for you. Try it again and step on the gas hard and fast. Either that or manually downshift into third.
1995 E420 - retired
1986 420SEL - retired