Couple of things could be going on here.
One your Surge Protector just decided to show you an issue that has been going on for a while and did not show up before. This is Highly unlikely but can happen.
Two running the AC on you 120VAC plug all this time has caught up and heated that plug to cause it to fail and possibly cause a short inside.
I run an autoformer that I installed inside the camper and run the AC and also watch the Voltage. Without the autoformer your voltage can and will probably drop to 109 or so with just the AC on and that really drives up the amp draw and then makes heat.
Next time you run the AC for a while feel your extension cable near the outlet and see how hot it gets. Even with the autoformer boosting it back up to 120 it gets warm.
I use the Hughes 30amp Autoformer
And I use this to monitor my voltage
Here is a thread I started when we traded campers on the install.
Traded Flagstaff 25KS for a Catalina 253 RKS
2015 F150 3.5 EcoBoost Lariat FX4 502A 3.55 Gears Max Tow KO2 Tires
2016 Coachman Catalina 253 RKS / Traded in Flagstaff 25KS
Recurve R3 WDH