It's not standard equipment on a lot of trailers, now-a-days. Often, though, it's included in the "Mandatory Option Package" that most manufacturers make you buy. If it's not in that package, many dealers include it in their "Dealer Prep Package". I swear, if the manufacturers & dealers had their way, they'd make you pay extra for brakes and lighting in the "Mandatory Vehicle Safety Package", and for a roof, walls, & floor in the "Mandatory Comforts of Home Package". Enough with the games!
Just build me a trailer I can go camping in! (Said the guy in the Eagle, watching the 26" LED TV, while typing on his lap-top, and sipping a glass of Pinot Noir)!