Cranking issues

Just bought a 1978 Chevy Van/Shasta motor home. It has a 350 and has been sitting a few years. I drained gas and added new, oil was changed about 500 miles ago (6 years) but looked really good. I cranked it with a little starter fluid and drove it about 50 miles home with no problems at all. Just tried to crank it again and it sounded like the starter was not engaging. No noise from engine, starter spinning rapidly with what sounded like no resistance. BUT, the belts were spinning. I was worried the timing chain was broken. I pulled a plug wire and was getting spark. So the distributor/cam was turning. Just sounded like the engine was spinning freely. I pulled a spark plug and piston was definitely pumping. Reinstalled spark plug and tried one more time. This time it sounded normal and cranked. (Rau rau rau rau vrooom) previously it was just (Rmmmmm)