I have a 2002 GMC 1500, for some reason the interior,dome door and under dash, lights are staying on. Although they will turn off when the 20 minute cut off switch cuts in

its either a bad door switch,or could be the dome override switch by headlight switch,or your interior light switch is all the way up

Same for both doors? If not, I'd check the switch(s) on the drivers door (may be more than one), or may be just one that has multiple contacts and the ones for the lights may be faulty.

one or more of the door switches are malfunctioning

Is there a button by the headlight control switch? I recently found out that the dome light only comes on while the doors are open when this button is engaged

The trigger for light is a return GROUND which goes to all the door switches and the light dome switch. so if there is a short on the holder or on the lamp return wire this willoveride the switches and the lamp will stay lit.
So remove the dome light and check holder and return wire to switches for short.

When dash lights don't work odds are the tail lights don't work either.Sounds like Dimmer switch may be a problem.

check your dome light override switch on the dash. make sure it is not locked into the "always on" mode. check you owners manual if you need help locating it.

Check under the hood, in the fusebox, for a fuse label ctsy (courtesy). It'll be over the driver side wheel. Replace it, if blows and, you have a wire touching bear metal somewhere. Good luck!

Lube your door latches. One of the door ajar switches are sticking . Use wd40 or some othe spray lube . Spray into the latch and open and close doors several time.