When towing my boat there is not enough power to go up hills. Even without the boat the power is not like it use to be. I have towed the same boat for 6 years and never had this problem of the engine straining on hills. I have a towing package on the van and I use it whenever towing.
Reduction in Engine Power on 2000 GMC Safari
Perhaps the fuel pump is failing and not being able to produce enough pressure or volume of fuel but you would need to tee in a fuel pressure gauge tape it to the windscreen and drive the truck under load to verify fuel pressure before just replacing the fuel pump. Check the air filter is not restricted a well.
Fuel pump was replaced two years ago, because of failure. Mentioning the fuel pump just reminded me that since it was replaced the performance has been decreasing. Could this be related? The air filter was replaced during a complete tune-up nine months ago. I checked the air filter and I do not see any restrictions.