For RVs there is a neat little fitting that is made to slip over the end of campground faucets that often have no hose bib threads. It it made of rubber and starts with a biggish opening and steps down to a smaller one. The other end has a brass male garden hose end. You can buy them from Camping World, Amazon, etc. It will fit on to most faucets and makes a decent seal.

You slip the Water Bandit over the faucet end and then either connect to your air pump with adapter fittings, or I just blow with my mouth to force the water back out.

You can do the same thing with clear tubing and tape.

David