6V PP translates to 4.242V rms (Vp-p*.707) across 50 ohms or .08484A (ohms law)
and the power is RMS voltage * current or about .35W not the nominal 6-10W
at nominal 13.8V.
I'd expect the RMS voltage for full power to be more than 17-20V RMS or about
25 to 29V P-P.
The other thing to monitor is the current of the radio during transmit. It should reach
2A (24-27W DC input) during normal operation.