Not sure about your make/model RV but some marker lights are designed to be accessed from the rear, i.e. inside the vehicle. Reach behind fixture, give the light bulb holder (not the entire fixture) a quarter turn and it will come out. Alternatively, it will be spring mounted, i.e. just give it a good yank.
When you have to replace the bulb from the outside, the lens cover can get "glued" on due to wax, caulk and dirt. So it seems you are "breaking" it, but it is just stuck on there. Sometimes you will break it for real and have to replace the lens.
If someone else has the same RV you have, they can probably give you better information. If not, post a closeup picture of the light fixture in question (with the lens off on the one you opened). That should enable someone here to give you information.
If the bulb seems not as bright, verify you have the same bulbs. Did you check the part number on the bulbs and replace them with the same? You can have bulbs that look the same, but are not.