Potty tongue --
Seriously, I don't think you'd want to put a potty on a slider. I don't know what size or type of potty you're talking about, but my 5+3 gallon model gets HEAVY. I realize that a loaded cooler is heavier, but I guess it depends how far you intend for it to slide out the back. Plus, show me a potty and I'll show you an eventual leak (pun not intended). Do you really want that in your galley area?
I would put the potty in a shower/potty tent. You can get them as cheap as $30 and they make perfect portable outhouses. When I go camping now, the potty goes in my tent, but it's a 12x14 tent with an entire mesh roof. I've been thinking about getting the shower tent myself, because we frequently camp with neighbors or extended family, and (of course) none of them have a potty for the kids in the middle of the night. It would help the whole group for me to put my potty in a shower tent. Of course, I'll have to do that once I build or buy a tear, right?
But I think for the drive I'll stow the potty on the tongue so I can tell everyone that I have potty tongue when I go camping. They'll just think I'm telling R-rated jokes around the campfire....