I second potentially too much timing for pump gas. If plugging the vacuum advance or retarding the timing were to help, it would more than likely confirm it. If no change then probably not the cause. If you get too much timing in the engine, especially with vacuum advance, cruising at 3,000 rpm’s on the highway could be firing your plugs off as early as 55-60 degree BTDC. As the plug flame spreads and combustion pressures rise and peak before TDC, it’s akin to that cylinder trying to kick back the entire engines rotation - thus the stuttering miss sensation. At 3,000 +/- rpm’s cruise your centrifugal advance may be fully in at the same time your vacuum is high and its advance all pulled in - everything advanced. Give it gas and the vacuum advance falls out reducing total advance and the engine accelerates smoothly.
However, this probably has little, if anything, to do with off-idle, low speed trailer hitching, especially if it has a pretty big cam.