If you dry up before you can really get rotting you will be pretty well preserved. The bottom line of that being that if your corpse is well-prepared before burial and your casket happens to be either well-sealed or otherwise escape water penetration for the period you are buried you will be in pretty good shape 300 years on.
There are a lot of ways a dead body can stay dry and a lot of dead bodies out there to discover.
There are also some other conditions that can result in surprising preservation, such as alkaline conditions leading to saponification (basically turning body fat into a soap-like substance).
There are a lot of ways a dead body can stay dry and a lot of dead bodies out there to discover.