Diagnose an engine "skip" under load

[keraf]

93 5.7 C1500 193k miles: On the highway pulling a 1 ton trailer and typically under a load (e.g. climbing a hill) my engine will periodically intermittently skip - a definite abrupt loss of power for an instant. I've only noticed this while pulling a trailer and under load. I've checked and cleaned the Distributor cap, rotor and PCV valve; checked the plugs & replaced the fuel filter. Fuel pump was replaced about 4-5 years ago.
Out on the highway without the trailer, I've not noticed this happening at all.

Additionally, on a trip last week, this behavior occurred and also my first tank full gave me 15 MPG and the second one on return gave me only 12 which is far less than I've ever gotten before. I usually avg between 15-17 when on the road with the trailer.

Since this is load related, I'm not expecting an ignition problem but something else.

Anyone had similar behavior and can offer any suggestions? Thanks.