When you gotta go, you gotta go. If there's no place outside the bathroom to tie a dog or even if there is, if the dog would not respond well to being left alone or approached by strangers, it seems logical to simply keep the dog with you if there's no time to get to the lavvy in the RV. When nature calls, folks don't stall.
There's also been problems with dogs being stolen as well.
Now if the dog is using the toilet, drinking from the toilet, or doing business on the floor, that's another story altogether.