If you replaced the whole marker light assembly, you need to make sure that it is properly wired and that you used the proper Wire Connectors for the job and that you did not just use electrical tape to make the connections.
If you just replaced the bulb you need to make sure that it was the right bulb for your clearance lights. If you used the wrong bulb and turned on the clearance lights it could have caused a fuse to blow or damaged a light relay.
Since you say that you have checked all of the fuses and none of them are blown and you have double-checked everything I mentioned above the only thing I can think of that would cause your problem is there is an in-line fuse that may have blown or a light relay that has gone bad. There is no way for me to tell you where this in-line fuse or relays are located because their locations vary by type and make of RV you have. Depending on the type or RV you have may not have an in-line fuse or light relay at all.
I hope that this helps.
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