If you are talking about the side marker lenses and if they are the old style like mine are with incandescence bulbs.
Then you might as well get a couple of spare lenses and carry them with you.
They are cheap and fall off often for no real reason. You can glue them on but, the bulbs fail often too and then you can't change the bulb out. If it's not the bulbs then it is the socket getting weak and not holding the bulb tightly anymore. If it's not the socket getting weak then it's corrosion and a bad ground (they are grounded to the skin of the camper)
Dammed things are always broken in some way or the other. About 2-3 times a year I have to do a check and fiddle with them to keep them working.