Might be easier to replace the TV with one that has a built in DVD. I have the Insignia 28" with DVD purchased from Best Buy last year, but they are now discontinued.
Amazon has some other brands, e.g. https://www.amazon.com/Sceptre-1080P...6379363&sr=1-2
Of course you don't get good sound from the TV speakers. We added a Bluetooth speaker on ours and we also purchased a BT transmitter which allows connection of two BT earphone/earbuds. We like to use the BT earbuds when watching movies when the A/C is running.
There are wired HDMI to Coax available but they seemed to be more expensive than a new TV. https://www.amazon.com/single-75ohm-.../dp/B00HWIWEMU
Not sure if they would work in the Airstream. What ever you do, keep it simple.