Better Times, first let me welcome you to iRV2.com. We are glad to have you join us here. If you are referring to the type that uses the molding as part of the hinge and the door has a mating groove, the two will come apart if the door is raised to its maximum height. Usually, struts are installed to limit how far up the door will travel as well as assist in operation. You could have a stretched groove on either or both surfaces too. The plastic piece is to keep the door from shifting laterally in the groove; acting as a spacer to keep the door apart from each other. It is hard to say what the actual problem is without seeing it.
Mike, RVIA & RVSA Certified Master RV Technician
Amy, Dr. Assistant - Roxie & Mei Ling, four legs each
2000 Gulf Stream Scenic Cruiser 450 hp & 1330# torque
06 Saturn Vue, 06 Chevy Z71 4x4 & 2014 Corvette Z51 M7