When tv gets warm, a black rectangle fills center screen on every channel. Once the unit cools down, it's gone again. The tv is three years old and is in a 2009 fifth wheel.

You posted under Leviton 6124 programmable timer.
If this is not your timer, add a comment
Leviton 6124 manual:
http://waterheatertimer.org/images/6124-manual.pdf

If you have different timer, look at link below to help identify timer.
Leviton Timers:
http://waterheatertimer.org/Leviton-timers.html

Leviton 6124 timer instructions:
Timer comes with plastic protector over display >> remove protector to see display.
Large button is manual override.
After timer is wired, push override to make sure Load turns ON and OFF.

Timer has 3 small buttons located below manual override used for programming
Display is divided into 24-hours with am times on left and pm on right.
24 Hour day is divided into 1/2 Hour segments
Each 1/2 hour must be set for ON or OFF

Start by pushing left-side event-cursor-advance-button and it scrolls down each 1/2 Hour.
Once you arrive at 1/2 hour > make decision if you want timer ON or OFF for that half hour.

Press event-set-reset-button in center to set ON-OFF for that 1/2 hour.

Press center event-set-reset-button and solid black rectangle fills in for that half hour.
Solid black rectangle means timer ON for that 1/2 Hour.

Once solid square is selected, press event-cursor-advance-button and cursor advances again and starts filling in black rectangles for each 1/2 Hour period.
Keep filling in black rectangles for all times you want timer ON.

When you reach 1/2 Hour period you want timer OFF, then press center event-set-reset-button so that no black rectangle is visible for that 1/2 hour.
No black rectangle means timer is OFF for that half hour.

To set current time, use right-side time-advance-button.
Press and hold time-advance and time cursor advances down through the am and then pm hours.
Stop cursor at current time, and timer is ready.

Is closed caption turned on? (and the channel doesn't have any captions) Turn it off.

Leviton 6124 timer instructions:
Timer comes with plastic protector over display >> remove protector to see display.
Large button is manual override.
After timer is wired, push override to make sure Load turns ON and OFF.

Timer has 3 small buttons located below manual override used for programming
Display is divided into 24-hours with am times on left and pm on right.
24 Hour day is divided into 1/2 Hour segments
Each 1/2 hour must be set for ON or OFF

Start by pushing left-side event-cursor-advance-button and it scrolls down each 1/2 Hour.
Once you arrive at 1/2 hour > make decision if you want timer ON or OFF for that half hour.

Press event-set-reset-button in center to set ON-OFF for that 1/2 hour.

Press center event-set-reset-button and solid black rectangle fills in for that half hour.
Solid black rectangle means timer ON for that 1/2 Hour.

Once solid square is selected, press event-cursor-advance-button and cursor advances again and starts filling in black rectangles for each 1/2 Hour period.
Keep filling in black rectangles for all times you want timer ON.

When you reach 1/2 Hour period you want timer OFF, then press center event-set-reset-button so that no black rectangle is visible for that 1/2 hour.
No black rectangle means timer is OFF for that half hour.

To set current time, use right-side time-advance-button.
Press and hold time-advance and time cursor advances down through the am and then pm hours.
Stop cursor at current time, and timer is ready.

If it is a large black rectangle that blots out the middle of the screen then you need to check your CC (closed caption) setting. Make sure the closed caption is turned off. I'm not up to date on the newer sets, but a lot of the older ones had more than one CC setting. One setting ran the captioning across the bottom, another put it in a big black box in the center of the screen. Most places didn't support the full text so all that showed was a big black rectangle on the screen. Since I'm not sure exactly what your rectangle looks like, this is my best estimate of your problem. Hope this helps.

turn off close caption

Hold in vol.up and vol. down at the same time. OR Channel up and channel down at the same time for 5 seconds. If you still have the same problem.Go into the menu and goto cc or close caption or text and turn it off. One of them should do it.

Try turning off the closed caption and text also check v chip, if you have a channel or channels blocked it could cause a black rectangle to pull up on the screen, if that doesn't work try resetting the unit to factory settings

Try holding the stop button and the DVD dubbing button at the same time and waiting about 3 seconds. I had to do this twice, but it solved my problem.

Go into your setup menu and turn off closed captioning. The box is there for some channels even though no text is being sent. If you need to keep the closed captioning, try selecting a different mode for it.

My mother-in-law had this problem on a Quasar for two years before she finally mentioned it. The rectangle is a text box for closed captioning or for a demonstation list of features, but without the text. I restored the factory presets and poof... problem solved.

Look in the manual and find how to restore the factory settings - it will probably be some weird combination of buttons that you would never guess. On her set, which has a built-in VCR, it was simultaneously pressing "play" and "FF" (on the set, not the remote) for five seconds until the set powered down. Yours will probably be something different but equally odd. That should get rid of the black rectangle. Then you go through the initial setup and you're back in business.