Locate the Bird relay and check that both large terminals show the same voltage on your multi-meter with the engine running ( may take a couple of mins if the chassis batteries are low.
OR, using the in coach battery voltage display , turn it onto, read house battery voltage after the engines been running for 2 mins.
Info on the system , in this file . Both the control box and relay are on the back wall of my power cable bay ( your's may be covered by a panel .
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.