I pulled the coax plate off the wall and had a peek around in the cavity with a flashlight and mirror on both of my Jayco's before mounting the BR tv. In both cases, I was able to slide a tape measure into the cavity to get an exact measure from the edge of the coax plate hole to the backer plate / stud mount location.
Ode to the Forum Moderators
May the Mods grant me the serenity to accept the opinions of those I cannot change,
the courage to offer valid input to those who are open to another way of thinking,
and the wisdom to shut myself down whenever I get too in love with my own misconceived brilliance.