85 suburban battary wont stay charged

i have a 85 suburban that i have replaced the battery and ault with new ones. i was told the old ault was only putting out 12.9 volts but when i installed the new one it is still not keeping the battery charged. it will slowly drain with only heat and head lights on at normal driving. is the votage regulator inside the ault on this year truck? thank you for any help on this matter.  btw im marcus a newbie here, thanks for having me and all your info on this board

When you say a " new " alternator do you mean rebuilt ?? It's not uncommon to get junk rebuilds.
 
Yes the regulator is inside the alternator.

yes it was a rebuilt one . im going to take it and have it teested today to see if its any good. my voltage gauge although may not be working properly(not sure) is showing 13 at best and with heater and break drops down to around 10 + or -

ok this ault was bad also , going to put on new one in a few. but i think ill have this one checked before i leave store, that way ill know if its junk or my burban burning them up.

ok , bought a new one had it tested at the store and it has begain to do the same thing as the rebulit ones. so im thinking my truck is causing this.. any suggestions? please...

Well there's only two things left : the cables and the battery.