GM-GMC: 92 GMC K1500 Truck 4x4 5.7, emission inspection, truck 4x4

Question
When towing my heavy camper, going up a hill, sudden backfires and hesitation when trying to accelerate. If hold gas pedal steady it clears up. Never does this any other time. Has happened last 2 times we went camping in EXACTLY the same locations? Not an immediate problem, I travel approx 20 miles or so before the problem shows up. Truck was just tuned up for PA Emission inspection. Am I looking at a fuel pump issue? Thanks in advance for any help you can offer.

Answer
My first suggestion would be to take a different route to your camp site.  Aside from that there are a number of possibilities, you can check your base timing.  Often technicians will retard the timing in an effort to help a vehicle pass, you may not notice the difference in timing until you really need the power (pulling a trailer)  The second could very well be your fuel pressure although fuel pressure problems don't generally happen only under load, they will happen randomly.  My first suggestion is to have a look at your base timing quickly and go from there.
Hope this helps