Sometimes bathing a dog too frequently, using harsh shampoo can actually make the problem worse. This strips the coat of natural oils which prevent any dirt & smells penetrating the hair shaft.
We have two big, hairy dogs, who like nothing better than rolling in mud, puddles, dead animals etc. We have never bathed either of them. If they have something noxious in their coat, we sponge the area in soapy water to get rid of the offending substance. Any mud etc gets brushed off when dry & hoovered up. The result? neither dog smells particularly bad. We have asked for honest feedback from friends & family & they agree with us about the lack of smell.
Anal gland problems can cause a pretty foul smell apparently. Get the dog checked out by a vet