On most toilet seats, the hinges have pop-up covers (rectangular or trapezoidal) you open up with a screwdriver. Once you've popped them open, there's a large slotted-head nylon screw that you can take out with the screwdriver. The nuts under the toilet bowl rim usually have an ear that stops them turning while you're taking out the screw.
Regular "round" (not "extended" or "elliptical") domestic toilet seats fit RV toilets.
Our seat lid broke during our spring trip to Oregon. I replaced it with a domestic seat from Lowe's - took all of 15 minutes to remove the old seat, clean off the top of the toilet and install the new one.