It might not have one the radio could be 24v, have you checked voltage at the radio?

G'day Kevin,
Do you know what else in your bus runs off 12v?
I had an '81 MQ Nissan Patrol diesel - there was no 12v converter supplied; they (Nissan) just ran the 12v accessories off one battery (not so nice). I built a 24v/12v converter which ran for several years until we drove across the Jardine River. Unfortunately my silicon sealant wasn't perfect, it filled with water & didn't work too well after that! I mounted it behind the RHS headlight for cooling reasons, hadn't banked on "water cooling".
So from then on, I reverted back to Nissan's original method. Only the car radio/cassette player & CB radio ran off this 12v feed. It worked for another 8 years until we traded the car.

I also recall a system of resistors that Hella produced at that time to "safely" split the batteries. I may have something at home still (stuck in Toowoomba with health issues). If I remember, I'll look it up later.