Yeah I know that Sears model. It had a two-knob tuner, no remote. The closed caption decoder was in a shielded box on the side of the chassis. It was a woodgrain-printed metal cabinet set made by Sanyo (I believe all Sears sets of that vintage were Sanyos). The date on the one I saw was mid-1980. I currently have TeleCaption I and II boxes which I believe are also Sanyo creations. The I has a 2-knob tuner and the II is remote controlled and cable-ready, channel display is red 7-segment LEDs. I believe those were also sold through Sears. I have a memory of seeing a I on display at Sears when I was a kid. There was a III which had the same features as the II but was scaled down in size, I've never seen one in person.
Is your RCA one of the models with the tuner built-in to the main board? (See here:
http://repairfaq.org/sam/rcatune.htm) Those are notorious for developing cold solder joints in the tuner which cause all sorts of havoc (bad tuning, no sound, etc.), but if you touch up the ground solder connections up they will run forever. I used to find those thrown out, would redo the ground solder connections and they would work perfectly afterward. If you start seeing symptoms don't let it go too long before fixing it, eventually they will start reprogramming their EPROMs and you have to replace the chip at that point.
RCA had a lot of innovative features around that era. I had an RCA with really cool PIP, you could move the PIP window anywhere on screen with the menu buttons (not just a few preset locations like most PIP sets).