The model number you submitted indicates your Roper dryer is electric and not gas as you indicated.
First, if you run the dryer with the door open it will cause the heating element to cycle OFF prematurely. Inadequate airflow through the heating element and heating element housing will cause the High limit thermostat (key#15) to open and shut the heater off due to the lack of airflow. Running it with the door open can also cause the thermal cut-off (key#6) to open and shut off the heating element. If the thermal cut-off opens, the thermal cut off will have to be replaced.
Generally speaking, when I dryer takes a long to dry clothes is a good indication of a restricted venting system. For testing only, remove the vent hose from the back of the dryer and dry one load and ff it dries the clothes in a normal amount of time that would confirm a restricted vent or too long of vent system.
I added the wiring diagram and the parts diagram in the image below. The heating element operation/temperature is regulated by the operating thermostat (key#24). No other component should control the heater operation unless there is lack of airflow or a venting issue. The thermal fuse (key#25), the hi-limit (key#15) and the thermal cut-off (key#6) are for safety in case it overheats due to a restricted vent or if the heating element shorts to ground and donâ€™t cycle off.
The most likely failure with your dryer is nothing more that a restricted vent system.