Did you purchase it used?
If so, did the previous owner install LED tail lights?
A fast blinking rate is a common issue when installing LED tail lights as they do not require as much current, which does not load flasher enough.
Incredibly, some of these vehicles still use the 100yr old mechanical flasher technology.
There are two possible solutions if a previous owner installed LEDs.
You can upgrade to an electronic flasher. However, some vehicles may still report a burned out bulb in OBD even if you are using an electronic flasher. If this is so, you may want to add a resistor in parallel to the LEDs (there are resistors made specifically for this).
The only thing that works on a RV is the owner...