My jayco has 12 v fridge didn't use microwave just ac it melted adapter between surge protector and ext cord .what I don't understand is neighbors have 28 ft coleman using same setup 15 k ac and standard fridge and no problems
The reason your adapter melted is because of high current along with high resistance. High current isn't necessarily a number that can be specified though as even a single amp or even less of current can become hot enough to melt something (consider a .1 amp (12 watt) soldering iron for example).
High resistance is the real problem. Often the adapters are made out of cheap materials and even those made out of good materials can oxidize the plug and outlet contacts. While you can't see (normally) the contacts inside the outlet you can see the plug itself and if it is not shiny new looking then it will have ever increasing resistance and this resistance causes heat.
What I like to do every so often (once a year perhaps but I don't base this on time) is to use a steel brush, sandpaper, or even my pocket knife (whatever I have handy) to clean the plug (all plugs, main cable and any adapters) so that they are shiny new in appearance again and then I apply a light coating of dielectric grease to keep (slow down) the plug from oxidizing sooner. Cleaning the plug is more important though than the grease as not every time do I have dielectric grease handy but I clean the plugs anyway. Even when doing all of this, it is often not possible to clearly see the contacts inside the outlet so that could still be an issue that will generate heat.
The adapters that are one piece seem to be more vulnerable than those with a short section of cable between the male and female connectors. While I have different adapters I prefer to use those with a short section of cable as that allows to some extent a bit of cooling between the male and female sides. With all of this in mind, a cheap molded adapter may simply just be of poor quality and have high resistance inside that can't be seen.
You have to be careful with this, but what I like to do (in particular if I have any concerns such as with an older adapter or outlet) is after I am setup and using an adapter (even sometimes when not using an adapter in particular if the outlet appears worn or damaged), after a few minutes of running whatever I plan to run in the RV (A/C for example), and again after more time has passed (~30 minutes or more), is to check all along the power cable connections for excessive heat. You can do this by carefully touching the insulated areas of the adapter and the plugs and outlets, or and as I prefer which is much safer, is to use a laser pointer temperature gun and if there are any hotspots found then you need to correct the cause of that (use a different adapter or clean the plugs for example and check again afterwards).
The reason your neighbor with the same setup hasn't had any issues is that his connection doesn't have the same high resistance that your connection (adapter) had. ~CA