My Dogue is the same on wheat based foods like Uncle Albers. Mine almost went bald. His skin pretty much fell apart, cracked, weeped and he stank in the space of a week and a bit. I had to malaseb him for ages to get him back to normal.
Never seen a Dogue do well on a cheap diet only, or one containing high wheat content. If they were doing OK on what they had why don't you cook once a week instead, or freeze big bags of it once you made it up. I just use the Vets All Natural now for 3 dogs and turns out cheap, and no cooking required.