In the past, I had the same problem with an Achilles. I had a garage to utilize at the time but any covered or enclosed area would work I can imagine. Point being the inflatable must be out of the elements. (Sun and rain)
I used a 50/50 mix of dish soap and fresh water and a paintbrush. I "painted" the mixture on a section of the raft and observed over several hours any bubbles that might have formed. Some bubbles will form initially however, with some time the area in question will develop larger bubbles revealing the leak. I know several people who used this method after I recommended it this summer with success. Important tip: Do not let preconceived ideas of where the leak may be lead you to ignore any possibilities. Even areas that appear normal and not damaged might have weaknesses that cannot be seen with your eye.
Bill, having a dinghy that is "72" dinghy years old doesn't mean she is tired yet but the light is close at the end of the tunnel unfortunately. Good luck my friend and let us know how you make out.
It's all Good