When tightening the cap or other fittings make sure to use a second wrench to hold the junction being screwed onto.
To test seal after applying cap (or any propane
fitting) use dish soap and water to paint
the joint with the propane
tank on or compressed air at low pressure hooked up. The soapy water will blow bubbles if there is a leak at that joint.
If you haven't yet tested all your fittings for leaks
with soapy water then you should plan on doing so. Just work your way back from propane tank and mop some very soapy water on each joint.
If you are really uncomfortable with propane put your cap on that open line and take it into a place to have it tested. Let them tighten the cap. Will make them feel like they accomplished something for you.
Places that sell propane to houses for heating. They typically also sell propane appliances and work on the lines. Have them test it. Camper repair places will also test but I'm betting the local propane supplier will be less expensive. Any heating and cooling shop or plumbing shop that does gas lines would be another source of a professional leak testing. Watch them do it once and you will have the process down to do it yourself from then on.