1999 Chevy Suburban voltage ok and drops with load

Let me start with I have had 2 certified mechanics 1 certified electical mechanic try to fix and my self with little experice but some experince of my own.
any ways just for kicks
car voltage meter on dash began to flucate from 14volt-12s with load ac/lights/radio, and put in revers
had battery and alternator checked at local parts store with meter said batt looked alittle low but was good drove about 5 miles turned off while I went in to store then would not start.
battery origanally had burnt cell so replaced
i have put 3 new batterys with over recomended cca, 3 new alternator, both pos. And neg. Post new, then new pos. Cable. From dealership to startor and alternator, check ground with jumper cable from neg. Side of battery to frame to check ground, ran jumper cable from fuse box /computer to pos. On battery no change in voltage reader says replace batt. Have taken off cleaned, inspected, and even replaced grounds for kicks, which I do know by experince this perticular local part chain has sold defictive parts but common 3 of each now I have to give them alittle criedit, batt. With car off 12.6 volts.
car on 13.9-14 volts, alternator with in.1-.3 volts from battery voltage with load ac/radio/headlight/put in revers to leave 12volts some times 10-11 volts based on dash meter if I have less things on lights only / ac only/ low 13 volts almost 12s

anyways any help appricated please forgive me if I dont donate at this time please under stand I have already paid atleast 3 certified pros with no avail but if I am guided in the right direction even from others on this page I will be more than glade to donate

thanks in advance for all the help and your time in reading this long post.