I just had a good scare with my 99 Cummins 5.9l, where the entire oil pan was caked front to rear with crud. I even had a significant amount of crud on the radiator. About half of the pan bolts were wet with oil, yet no dripping.
I thought I was really in for a $$$$$ repair.
I had my mech change the pan gasket with an OEM Cummins gasket and change the oil. He also felt into the torque convertor housing at the bottom and came up with dry fingers. After some driving, he looked and found that the pan gasket was the failure...NO MAIN SEAL LEAK!!
I have also put a plastic gatorade bottle over the crank case vent tube and tie wrapped it to the chassis so any oil that comes out of the tube goes into the gatorade bottle. I think this was one factor to such a horrendous amout of oil buildup, along with the pan gasket.
You would do yourself a huge favor to fully wipe your pan down where it meets the block, fully. Clean that thing so you can eat off of it and drive your coach for a while. Monitor your oil level and even if you're losing oil, keep it topped off for the test period and take another look at your pan, front main, rear main for ANY sign of oil. If you find it's not your pan gasket, work your way up from where you get oil witness staining. This should be easy to work out, if you're patient.
Don't just take their advice. Even if you have to spend a little bit of money to prevent that huge rebuild cost, it's money well spent. IMO