Steve, I put a level on my head and it was a quarter bubble off. What does that tell you about me? lol Have to get my money back from that flat top haircut.
Although it is tricky to work with, he could use some type of floor leveling compound or what I think is easier depending upon the scenario is some thick set mortar. I don't claim to be the floor expert but there are a few around who haunt this site. It may be a matter of putting down what you want and live with the slope just like many people in mobile homes. I'd sure try a floor leveler first though.