Open all windows and vents, open cabinets ... turn all vent fans on for a couple hours overnight if its convenient. Then close it up for a few hours, or overnight. If it smells like gas again, you probably have a connection leak inside.
Lots of soap bubbles on every joint - watch for the bubbling. If that don't fix it, put plugs in the unit connectors and put a ten pound air test on it, and soap the joints again.
They sell test fluid, but it's not any better than bubbling dish soap, and costs more. Good luck.