Losing both side markers is an indication that there is a power interruption.
Bad fuse?- yes, but more likely a bad ground or shorted power wire.
Do you have a circuit tester?
https://www.amazon.com/OctagonStar-C...002192&sr=8-22
With lights off, Clean both light holders (sockets), and give them a good look to make sure the wiring is intact. Not broken or rusted.
Check for 12 volts dc by placing the tester probe in the socket and the tester clamp on to a good clean ground.
Turn the headlights of the coach on. If the light in the tester glows, it means there is power getting to the socket.
If only one of the sockets shows power, or if neither socket shows power, clean the grounds at both sockets.
If you an access the wiring, look carefully for a broken wire.