If it was my dog (and I have recently been in the "OMG the dog won't stop scratching" stage). I would;
Put it on cooked chicken and rice for a bit and get see if that helps. If it does - your problems are probably diet based. Slowly re-introduce foods one at a time and keep an eye on reactions. If not - blame the environment.
Do other things as suggested above + wash / replace all bedding.
Look at bedding - what is your dog sleeping on? The majority our dog bedding is cotton or wool or towels. Washed very often in sensitive washing liquid (no perfume, low allergen) and HOT water.
Flea bomb the house (even if you can't see fleas)
Wash your kennel out with warm soapy water
get some fresh air and sun into your dogs environments.
Good luck. Having recently been through itchy, smelly, fussy dog territory I feel your pain.