We just returned from our first trip out with our new 2016 31FK. All systems worked as they were supposed to including the hot water heater, no vibration on the road and it behaved like a dream. Still have some storage space to fill.
Hello Shabti. No the mattress is 60x 76 with rounded corners at the foot. If you put in a 60x80 you would need to add support at the end. However then the clearance would be very tight and the new corners would be in the way when going around to climb in. We found that it was very doable for us as is. Like Greyhawk01 mentioned we love the tv locations. Hope this helps you a bit.