Yes, you can use the truck bed battery set as "downstream" batteries in parallel with the camper set and run the rig's load/charging wires from/ to the camper set via the Tri shunt. It is not ideal but not a big deal. Not worth the huge bother doing it "right" for the amount of benefit. ( I have the same situation with extra batts in the bed and more batts in the TC. BTDT)
You can still make the truck bed set itself balanced, running the pos and neg links from that bank to the other bank from opposite batteries in the bed.
