connecting new tv to Direct Tv

Your DirecTV box only has Red/White/Yellow outputs, nothing else? What is the model number?

TV only has the coax antenna input, nothing further, not even HDMI?

If that is the case, you have an outdated setup that is shutting down. SD-only TVs haven't been made in over a decade. DirecTV discontinued offering the related service back in late 2015 and is now in the middle of shutting down the MPEG-2/SD-only feeds.

You would need to call DirecTV and request a MPEG swap. This is complimentary (no cost for equipment or a new service agreement), as is required to continue using service. Though read order confirmation to be sure it was not placed as an 'upgrade'. Let them know to put a note on the order you need an adapter as you have an ancient TV (if they provide them anymore). Otherwise, you should see about getting a simple HD TV. They are no longer supporting legacy equipment.

I don't think DTV ever supplied RF modulators.

Some techs had them on hand, but I don't think it was a requirement by DirecTV itself they carried them.

Unless the OP is mistaken about the capabilities of their DirecTV box and/or TV set, they are well overdue to get equipment swapped. If they post back with models of both, we can be sure.