The typical AD/DC clampon ammeter measures voltage with the probes when it's set for DC voltage. When set for DC amps it measures amps with the clamp on one wire. Follow the manual instructions.
It can take several days to charge the batteries with that small panel. With the panel aimed directly at the noon sun expect 4A with a discharged battery. This is with the batteries disconnected from the RV.
Disconnect the batteries and wait several hours. Then 12.7V for both in series is fully charged and 12.1V is 50% charged.