Likely the peeing was due to the pred. It's a common side effect.
We don't currently have a dog on pred, but we have a few that are suffering from spay incontinence that occasionally break through the meds and 'leak'. We have what are called "oops pads" on our beds--they're waterproof, but have a soft side for the dogs to lie on. We also have waterproof mattress pads on our beds--that way if they miss the oops pad, all we have to do is deodorize and wash the bedding, not deodorize the mattress, as well. It will at least make things a little easier and cut down on your laundry load...
I haven't had any experience with mast cell tumors, so have no specific advice. There are a lot of people on the board who have experience with cancers, in general, though, and may have more advice for you.
Meanwhile, many of us have lost a pet and we're here if you need to rant, cry, reminisce, ask questions, or just need a hug.