Large performance drop 1997 7.4

[spenc938]

My parents own a 1997 GMC Suburban with the 7.4. It's always been a great truck, lots of power, but when we took our trailer out this year it has no power at all. There is a ten mile long hill on the way to a place we go camping every year. We typically go 60-65 no problem, but this year the 'burban struggled to maintain 30. Another problem we noticed was that if you turn on the hazards when towing the trailer, the entire truck will shake pretty violently.

A couple weeks before we went camping I replaced the plugs, wires, and fuel filter. I'm thinking that maybe the truck isn't liking the new plugs. What do you guys recommend as far as plugs? I like to use AC Delcos, but at the time I was overruled by my father. I don't remember for sure which ones I put it, but I'll pull one within the next couple days and edit this post.

Edit: They're Autolite Platiums

Edit 2: At idle, fuel pressure hovers around 55.