Re: ATF: Sundance - Sundance trailer is a fire hazard
You said this was a Sundance in your title, but your current rig is shown as a 2008 Heartland 3670 (which is a Bighorn). At any rate, it does appear that the cover was left open over the water heater exhaust while the propane was firing. With the lightweight plastic and the heat that does come out of there, it would not have taken long to cause the damage. I would agree, regardless of model or brand, that the placement of the shower over the water heater such that the thin plastic door could hang over the exhaust is not good engineering.
On our 2009 BH3670, the shower is inside the UDC and there is nothing over the water heater. Even the water heater "door" looks different. The door for the sewer hose compartment is, however, the same as your shower door. It even has winterizing instructions for the shower molded in it. But it does not have an indication of what type of plastic it is made from.
Hopefully, they've changed the mounting positions on newer models to forestall that from happening again. Unfortunately, that doesn't do you much good. Fortunately, nothing more serious than a melted door occurred. Perhaps the factory will weigh in on this, either publicly or privately to you, with some resolution.