Do you think it's right

[Carsman]

The thing is he's pissing all over himself and it smells like death. Plus he's 150 lbs so it's not like he's a small dog that I can just wash up in the laundry tub. He doesn't have a good quality of life because he can't walk, but he's not sick and in pain. He still acts like a happy dog even though he has to scoot on his ass to move. It sucks because I'd have an easier time euthanizing him if he was just laying there depressed and sick, but he's fine except for his legs.

Sorry for your "big" problem.
He's "Big", so of coarse that's the difficult part for you, having to "clean up a ton" of shitty-piss" every day! And it will only continue for the rest of his life.
Since he can't walk, his "quality" of life is next to non existent.
He won't be getting better, so it's up to you just "how long" you can put up with your furry family member's situation.
"15" dog years is said to be "105" human years, so it's about time.