My sister owned horses for many years. I'd help her sometimes at her place, and shoveled a lot of horse crap.
At one point, Dad used some to fertilize his plants, but we quickly learned to not use that which the horse had peed on. It'll burn the plants! Best was horse crap mixed 50/50 with storebought cow crap. Grew some great tomatoes!
After using her horse trailer, she'd always elevate the tongue and hose it out. Kept it fairly clean inside.