If the volume is adequate when parked, it's not uncommon for road/wind noise to over power the TV speakers. How about a set of headphones? Either tethered with an extension if needed, or the many wireless headphone models now available. The other option is to connect the TV audio to your radio for additional volume.
I'd opt for the wireless, noise cancelling headphones, then the driver could still listen to the radio while passenger watches the TV.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI