What your are looking for is DIN removal tools
. Crutchfields is a reliable place to purchase tools, radios and get advice. Bear in mind that these tools can be tricky to use at times and that you can also foul-up the release mechanisms on the radio mount if you aren't careful. Once out, measure the opening of the mount and then you can key on what size radio you need.
Crutchfields car stereo installation guide
Crutchfields can also likely help you get the proper connectors so that you can mate up with the existing harness. Might be a bit tricky as it is an Airstream and not a car. Having the correct connectors will make installing and removing (if necessary) the radio much easier than if you directly connect the wires. Just sayin' as I have been there, done that many times for many years.
This tread is a motivation for me to finally get rid of that Audiovox in mine and replace it with a decent (likely) Kenwood or Alpine. As mine is not a standard mount, I need to install a DIN mount in a wood "face" that I can swap out for the existing wood mount. The hard part will be trying to get the wood to match. It is possible I will go with a "double DIN" mount since I have the room. That allows a larger control face for the radio. I'll have to see what is out there.