They are easy to replace but some people have a time getting to it. It's usually up behind the sink bowl. They have to be at least 6 inches above the height of the toilet bowl. They are usually black and can become stuck open allowing water to leak and a sharp bump can dislodge the stuck flap so you don't need to replace them. They are directional so be sure you install it the correct direction. The first picture is a common one and some people replace them with a brass one like in the second picture. The black pipe to the right in the second picture is the air admittance valve for the grey water tank, they sometimes stick so the sink doesn't drain properly, a rap with a screwdriver handle will free it.
Richard & Jill
2014 Flagstaff 832IKBS Classic Super Lite
2018 Silverado 1500 Crew Cab Z71 4WD All Star Edition
Camping since 1989
Car Shredder Op/Tech, Scrap Metal Recycling - retired
I deserve hazard pay when camping with my relatives.