Are you asking about
both the coach stereo and the cab stereo?
Regarding the coach stereo, if you have the Jensen JWN6A head unit in your RV, then you will find that it comes out easily for access behind the unit. There is a lot of space back there to access the wiring. Notably, you have access to the roof panel of the RV and should be able to see where the coax cable for the tv is run. On my rig it is simply a hole with the cable through and a whole lot of sealant to fill the hole. I suggest simply running another coax for whatever antenna that you want to mount using the same hole.
However, you shoud note that the fibreglas & foam construction of the rig is transparent to radio waves. I am currently parked right beside the satellite and SiriusXM antennas that feed my house. The signals pass through the RV unaffected. I suspect that the antenna on your cab is also unaffected and that you are simply in a poor signal area. Did you try reorienting the RV to check the signal?
To identify, here is the Jensen JWN6A manual:
http://www.forestriverforums.com/for...0&d=1469648393