I have two litter boxes. One has the veggies in it and the other has the pellets and hay. My resident pig, Timbit, uses his boxes about oh, 90% of the time. I find a stray poop or two around the edges of the boxes and in his fleece hidey, but otherwise, he's very consistent and I'm a happy camper. When I change out all the bedding, most of the fabric is dry except in his hidey and right next to the veggie litter box (ha, maybe he wasn't eating those times and is too lazy to jump in and use it).
The pig we just adopted, Pumpkin, who is in quarantine right now has one litter box with his veggie and pellet dish. He uses it haphazardly. The rest of the time, he's been pooping in his pigloo and the corner. There's also urine throughout the cage. I've been trying to head him off with a corner toilet, but every time I move the toilet to his favorite pooping corner, he switches it. I can only hope that once he's in the 2x5 C&C he follows Timbit's lead.