How many hours on shoreline are you charging them? Driving a couple of hours won't do it. You can get full battery voltage without them being full. This surface charge will bleed off quickly and give you the lower voltages you are seeing. Use your friends clamp on meter to measure the charging current going into the batteries and then the math can be done. The condition of your cables is unknown to us as well. Most batteries in RV's are murdered and not given a chance to die a normal battery death. Poor maintenance and lack of understanding contribute to the issue.
