does this suggest my shunt is wired up back to front.
only Dometic cr50 running and background 12v circuits connected but not on.
You are drawing power out of your battery.
If you have no loads on and it is supposed to be charging or is charging, then it is backwards or on the incorrect terminal as suggested by @Netrange.
The other possible issue may be there is a short/load on that you are unaware of. This means wiring issues.
And Starter battery 0,03V indicates something wrong.
