Unless somebody has siliconed them in place, they pop off.
I recently replaced the lenses on all 13 clearance lights on our rig. After visiting every RV store in the Mesa, AZ, area, searching on line, and getting a local dealer to call the one guy working at Monaco National Parts, I was unsuccessful in finding the exact same lenses. I ordered the lenses that Monaco said we have from an online dealer, and they were close. I had to take a Dremel to each one to get it to fit. There are infinitesimal differences among the hundreds of models out there.
One place I didn't try was a truck stop. Somebody told me that they generally have on display many models of lenses.
It sure looks pretty now with those bright, unfogged lights, though.