The IBC may not put out full voltage sitting still so I would take the truck and IBC out of the equation. Do not hook the truck up at all to the trailer.
The breakaway switch it nothing more than an on off switch. If you pull the pin the connection is made. This is a simple 2 minute test that should tell you if you are getting full braking.
Again truck disconnected from the trailer as in no umbilical connected.
Pull the emergency breakaway pin. With the pin pulled take a voltage reading at the battery. Then take a voltage reading at each wheel. The wheel voltage should be within a tenth of a volt or two of the battery.
Plug the breakaway pin back in to stop draining the battery.