The grey goo could be self-leveling caulk, a common sealer for seams around rooftop vents and hatches.
The tires are an expensive item, normally on an RV they get old before they get bald. If they are older than about 6 years, they will need replacement soon. Tire age codes
Check that all systems work as they should. Also scan for stains and evidence of leaks. Dampness breeds rot and mold, both bad in an RV.
Engine and drive train should be inspected for neglect, as should brakes and other driving functions. Listen for exhaust and manifold leaks, oil and coolant, too. Drive it long enough to warm up to be sure the radiator cools properly.