[travelin texans]

I would recommend NEVER use a snake or ANY drain cleaning chemicals in any rv, you'll end with way more damage than you'll cure!!
Have you taken the drain apart? The factories are notorious for lack of construction clean up, you may have a drain full of saw dust, a rag, screws or maybe a screwdriver. My friend took the underbelly lose to reattach a drain cable & found a Dewalt impact driver.
My guess is the tanks full & you haven't found the handle yet.