You could also be reading the corrosion between body and frame or something like that. A digital meter doesn't put much load on things so they will practically read the voltage just from the air. A potato likely has a more amps (and volts) in it than you are getting from your truck.
98 Burb K25 4.10's 250K miles, new to me, pretty clean and stock.
95 Burb K25 3.73's CRASHED 250K miles, looks to have been rebuilt at least once, stock, bad rust, DE exhaust w/walker muffler, 8' western plow.
81 Buick Riviera GAVE AWAY with the infamous Olds 5.7 diesel possibly still going strong.
15 VW Golf wagon, TDI. Did the buyback on my 13 Jetta TDI wagon so it was basically a trade up.
96 Burb K25 3.42's SOLD at 340K miles, told it was all original, close race between engine wearing out or body completely rust out.