Stop in for a cup of coffee

so the cummins, each battery while running shows 15.6 and 15.8 volts. shut it off, let it sit for 20 minutes, went back out, put the multimeter back on, dropped as i held it on there, a tenth of a volt every few seconds. down to 11.3 and 11.9. Looks like its time to start pulling fuses and seeing if the drop stops