Compression testing seems like a fascinating exercise. But alas, I do not have access to the fancy MB tester. I see my local O'Reilly has a generic loaner, which looks primitive on the website.
My thinking is that I get good gas milage, therefore the engine is fundamentally in good shape?
Starting with the easy stuff, what about pulling off the rubber tubing from the vent barb, and check whether the port is plugged?