Its also possible to have a servo going bad produce the low reading on the meter.

Since you get the low reading with 2 tested good batteries... I would be looking for the problem n the radio system, not the batteries. If the switch harness checks out, then try unplugging one servo at a time. If that doesn't locate it... test with a different RX. Its possible for dirt and oil to get into a RX and cause problems. (especially in some RC combat planes and the "PBF" where the RX is not enclosed)