Owned the 2002 version of same coach for 8 years. Usual suspects in that area are engine coolant (hose, loose clamp, leaking radiator). Another possibility is hydraulic fluid from a leaking bedroom slide hose or ram. Obviously, should be easy to identify coolant or hydraulic fluid and you did not say it seemed to be either of those.
Since you were driving in rain water may have accumulated somewhere and just drained from under the coach. You said it was 'slippery' so that is curious.
I would just get on a creeper with a good flashlight and check everything over well.
