All of the TV's in our Seneca are Continu us models and we haven't had any problems. I did replace our antenna with a head unit that included the "Wingman" extension (I think that's what they call it) and it made a substantial difference in the out-of-the-way campground we frequent.
I don't really understand how a TV could have a "weak" tuner. Have you made sure that the coax connections are tight on those TV's? You might also verify that the Continu us TV's are set to "Air" and not "Cable" in the settings.
Ty & Deb Montag
2015 Jayco Seneca 36FK
2014 Honda CR-V toad