The valve is held in with 4 bolts w/ nuts. You may have to drop the belly pan to access them if there is no access panel. It might be near where you pull on the handle. I can't help with exact location as my MH has no belly pan. Check for leakage by putting water in tank while drain cap is off, don't wait for the drain pipe between valve & cap to fill. A tip is to lightly coat the rubber seal with vaseline where the valve slides. Don't overtighten the bolts. Snug them up & put clean water in the tanks with valve closed. Then evenly tighten all bolts til leakage stops.
There's also practical jokers out there. I've pulled into a site & had sewage flow out when hooking up. Somewhere at one of my rest stops the joker pulled both dump valve handles & closed them back up.
always remember to hook up slinky hose to site 1st. Then hold RV end under cap & loosen a little. This way you can control any unexpected sewage leaks & keep the ground clean.