I am trying to broadcast my home video surveillance video to all TVs in my house through the same Coaxial wire that carries my TV signal which is ATT U-verse. The U-verse receiver is connected to the wall and I have my TV connected with a HDMI cable.

Would it be possible to have an RF modulator connect to the wall as TV/ANT in and then the TV out go to a splitter. The splitter will connect to the U-verse box and I can watch TV with HDMI input and the other output on the splitter will go to AV in on the TV which would broadcast the signal from the input Video In on the RF modulator?

I have a video out from a DVR for my home surveillance so basically it's the same signal output as a DVD player. I spent several hours trying all different sorts of tricks to no avail. Can anyone offer any assistance?

You can use a 2 way splitter as a combiner, You need to program the Modulator to an unused channel, then feed the channel backwards into the splitter, then into Main splitter, You will need a Trap to trap out frequencies you programmed Modulator for.
Depending on where you insert the video, you may want to trap your video form getting out of your house, (Incase you had a Camera in the Baby's room), or an area no one else needs to see.

I'm confused. Explain?

Digital TV modulators don't seem to exist.

Are you trying to put the camera on the same cable as the cable TV somewhere else in the house and then extract it again at the TV outlet as RF and then change it back to composite video?