battery short

[turf]

i just replaced my starter batteries because of a weird voltage, but with the new batteries, the weird voltage did not go away.

my batteries were fine when i installed them.
2 days later (today) i started the bus for the first ime on these batteries and looked at the battery meter. all looked strong. 13+v, the alt was charging. looked good. it ran like that for 15 minutes.
i [pulled out on to the road and was cruising to an rv park, and 5 minutes down the road, i looked up again and my volts were 10.3 with the alarm going off.

at first i thought maybe it was the brake circuit, but that has little effect on the short erratic voltage.

the culprit that seems to trigger the short is moving the trans lever from neutral to drive. in neutral, the volts are stable. when i move to "D" the 10.3 volt and the battery alarm go off.

not sure where to look...... virtually an at545 trans.... really the big brother at1545 but there are no difference to the electric portion (does it even have an electric part?).

any tips appreciated

[flattracker]

My approach would be:
Using a clip-on dc ammeter measure current flow from the alternator with the engine running, trans in neutral. brake set. Harbor Freight sells them. Also check the voltage at the alternator. Then, keeping the brake set, place the trans in drive and re-check the current flow and voltage from the alternator. Do not leave the trans in drive for long as it can get hot. You should not have a change in voltage or current flow. Even if the alternator output goes away your batteries should not go down that quick. Have you connected any additional loads to the bus electrical system? If you have, disconnect them and repeat the test. If the results change then look at those circuits that you added.