Dometic comfort zone furnace turns off after 30 seconds

A bit more specific if it runs fore 30 seconds and shuts down (It should run for like 3 minutes) there is a LED on the control board (likely on the left outside) note if it is blinking 3 blinks pause 3 blinks repeated means "Ignition lockout"

Mine did that when the board failed to detect the burner was lit. oh the burner lit just fine but the detection chip was toast. New control board from dinosaur board fired 'er right up. No problem.

But you'd get the same indication if the burner did not light.

if it runs 30 seconds then shuts off, then the burner probably has not lit. The furnace then goes to lock out. Odds are the hi limit switch is stuck open. Take a hammer and gently bang on the outside metal exhaust tube for about 30 seconds. Then try to run furnace, if it runs and lights you need the Hi limit replaced. Doug

Does it try to light?

I am guessing the furnace is not operating.
Open furnace access, locate the two blue wires, install a temperary jumper between them. The furnace fan should start, furnace should start producing heat. If it continues to operate, remove jumper, furnace should shut off burner go into cool down and then shut off.
If it does the furnace is operating correctly and problem is thermostat or it's system.