Where can i find a wiring diagram for a norcold model de251e

Parts suppliers in the link

https://www.google.com/search?q=Norcold+DE251+replacement+thermostat

..

Most places are marking parts as obsolete. Try the first two, although one lists it as a power supply board and the other as a door retainer. Either way, you are better looking for a complete new unit.

Norcold # 618198 / 625386 2-way Power Supply Kit

https://www.partsfortechs.com > asapcart
Norcold discontinued part #'s 618573 & 618574 - and replaced them with this kit. NOTICE: Although we have basic RV appliance knowledge, our RV refrigerator ...

Norcold Parts for 962, 982, 983, 9162, 9163, 9182, 9183 (900

https://thenorcoldguy.com > norcold-900-9100-parts
Norcold 900 & 9100 Refrigerator Parts. Norcold 900 Models: 962, 962IM, 963, 963IM, 982, 982F, 982IM, 983, 983F, 983IM. Norcold 9000 Models: 9162, 9163, 9182 ...
Missing: #625386

9100 series - PARTS LIST

https://www.thetford.com > uploads > 2015/04 > Par...
PDF
PARTS LIST. 9162. 9163. 9182. 9182F. 9183. PART NO. 634012. NORCOLD 09/17/2014. 9100 SERIES ... CONTROL ASSY-OPTICAL (USE 625386 FOR EG2/625387 FOR EG3).
9 pages

The high temp switch connects to your power board at "Limit In" and "Limit Out". Follow those wires to the switch. I would see if you're under the Norcold recall. Give them a call and check. If you're not, here is where you can get an upgraded thermal switch:
Norcold High Temperature Limit Switch V2 0 by ARPrv

Sounds like your selector knob may net be making proper contact with electricity. You have changed everything else. You could check power available with a multimeter to your element. If power not available to your element, back trace to source and check for burned wires or breaks in wiring.
Hope this helps!

you can reset the control board . to do this you must ground the # 15 terminal with a jumper wire to ground. this is usually the terminal next to the white & violet wire. check diagram.

try blowing through the gas orifice with compressed air only.

u need to replace the thermocuple or be sure its near the pilot also be sure to blow out any and all dust in that area

I found this. Hope it helps. http://www.landonedwards.com/carter/files/Thetford-Norcold/TekIISM.pdf

Check if you are getting voltage to the heating element should be 120 volts