I had to replace my Axerra radio by a Pioneer. The only problem I had is when I would turn off the RV, radio was still running. So even with shutting it down manually we found out that it was draining my batteries. You may want to check this.
I went to a pro shop and they did correct this wire connection issue.
Without using the shore power you may want to consider using an intelligent charger.
That's what I did and my batteries are always full charge . House batteries in store mode.