For whatever reason, the industry is fixated on strip lighting right now. Chinese nock-offs are super cheap from flea-bay, aliexp, etc. The real deal cost a bit more, but still affordable. Plastic tape you can stick down, that's covered with LEDs in any color or shade of white you desire. High end strips have the plastic tape dressed up with an aluminum extrusion making it look nicer from the side.
12v illumination grade fixtures exist mostly in the form of recessed or surface mount lighting, like puck lights. Drill a 3-5" hole, snap in the fixture, bam. Lots of options here for brightness, color, focus etc. Either a simple flood or a fancy eyeball light you can aim.
RV "pill / oval" shaped lighting fixtures use typically indicator grade LEDs, replacements for the 921 halogen bulbs, etc. This is far less than ideal, although many seem content with the solution. Indicator grade doesn't offer the best in terms of color rendering (makes colors look weird / washed out), and some are not designed with continuous use in mind (over heating on hot days for example). Something called a COB light is a good substitute for the 921 type bulb. The COB gives you a "sheet" of light, you glue or screw into the fixture.
Designing something from scratch, my advice, avoid the pill / oval fixtures, unless you want to maintain the ability to go back to halogen bulbs. Puck lights and strip lights offer better light and better economy.