I suspect the adapter and extension cord have been connected together too often while the power is still on.
Many folks plug their cords in while live, instead of turning off the power at the source (CG pedestal, or breaker at home) or inside the camper. Every time you plug in live, there is a momentary inrush current caused by the converter. This is the zap you will hear, and in the dark, a flash as well. This causes pitting on the blades of a plug and inside a receptacle or inside the connector on the end of a cord. The more you do this, the worse the pitting gets. The pitting also attracts dirt. The result is that resistance rises over time to the point where it gets hot. It can melt things or even cause a fire. A meltdown usually happens at the pedestal end of the cord, but I saw a photo once of a burned out power inlet on the side of a TT. The owner was always plugging in at the pedestal first, then attaching his detachable shore power cord with the power on. He was lucky that it only damaged his inlet fitting.
When we first got a TT, I used to plug it in at home to keep the battery charged and fridge running on 120V. I ended up having two extension cords go up in smoke. I eventually installed a 30A RV outlet in our carport along with a disconnect switch below it.
You should periodically check the blades on the end of the shore power cord and clean with fine emery cloth or steel wool. There's no way I know of to check inside a recept. or connector.
The reason that the compact "puck" style adapters don't work as well is because with the short length of the adapter, you have prongs/blades inside the connector/receptacle end and blades at the plug end in close proximity to each other. If they are both pitted and dirty, there is more heat concentrated in a smaller area. With a dogbone adapter, the plug and connector are separated by 18" and there will be little heat transferred from one end to the other.
If you want to plug into an extension cord at home, always turn the power off first at a breaker. Secondly, use a heavy duty extension cord. A #12 gauge cord will not only have lower voltage drop, it should have a more heavy duty connector on it.
I've always eventually had trouble with the connector end of even heavy duty contractor grade extension cords. They always seem to lose tension of the contacts inside the connector/outlet and eventually make a poor connection. If you do get a heavy duty cord, you might consider getting a high quality plug and connector and replacing what the cord came with. You could use something like this Leviton connector (and matching plug) which is industrial grade with a lifetime warranty.
I have a 25' 15A extension cord I made up that I store in our TT and I used a Marinco connector and plug on the cord.