Any parts store will carry a functional gas cap for your rig, if it is an F53 chassis. Make sure to tell them it's an F53, and not a National.
2006 Damon Daybreak 3276 35'with 5 Star Tuner. 3 200 Amp Lithium batteries and 2000 watt PSW inverter/charger. 2013 Elantra on a Master Tow dolly.