Even a small oil pan leak, if it drips it will not hit the ground, and likely coat the entire bottom of the vehicle. I have seen this myself many times. That much oil in 200 miles, would rustproof most vehicles frames and you are correct not visable at speed. But should be a huge indicator that its time to get off the road and find a professional to diagnose the problem unless you are handy with engines yourself.
It really does suck, 15k for a new motor sounds high except its an RV so the labor has got to be about twice as many hours for the R&R vs. a car or truck.