Yes, a RF modulator will take your the red, white, yellow plugs and convert them to a RF plug, like you would use with an old fashion antenna. However, if the RCA unit is defective, out of the box, it won't work, and you may not know why. I suggest buying from a store that can show you a working unit hooked up in their store, if anyone does that any more. Good Luck
Yes, it would work but I have some better ways. I would suggest you unplug you the rca that you have going to your direct tv receiver to the tv and use that for your dvd player. The coaxial cable connecting your tv and your receiver has both sound and video running through the cable so you don't need the rca connection for that. but if you want to keep it that way I would suggest a RCA 4-Way AV Switcher With S-Video that will give you more rca connections if you like to connect other thing to your tv all you do is press the button on the top of the box to switch between dvd player and direct tv . here is the link just click on it to take you to the page to see how it looks like. http://www.walmart.com/ip/RCA-4-Way-AV-Switcher-With-S-Video/3312963
I have an old tv connected to the compact rf modulator and a unit that records both tv and dvds and is a dvd player. I would say that u don't need anything else to connect your rf modulator, except u stated u use the cable with the red, white, and yellow plugs to connect your Directtv receiver. U may need an additional one of those to connect your rd modulator.