With the photos I can see exactly what you mean. That cap for the city water is made of a much more pliable plastic than the FW cap or the actual city water attachment. It deformed where the others didn't. A poor design for sure. When the cap is removed for use it would hang right over the exhaust vent. No wonder it got hot enough to deform. It wouldn't take much.
I'd see what the dealer or Keystone has for a fix. I've had good luck contacting Keystone Owner Relations:
https://www.keystonerv.com/owners/contact-us/ (I used the phone #. They will answer and did follow up.
Keep us posted as there are likely others with this same set up looking for an answer.