On our coach/set up the impact on the coach mileage is negligible....maybe a 1/4 of a mile per gallon at the most. I don't even know our jeep/wrangler is back there.
We rarely RARELY go anywhere w/out towing our Jeep....its good insurance, if we were to break down someone can run for parts or i can send the wife ahead while i wait for road side assistance.
I put a bike rack on the back of our Jeep & carry the bikes there.
FOOD FOR THOUGHT: On a (local) camping trip my wife fell & broke her arm....it was much easier to run her to the hospital in the jeep than it would have been to disconnect the coach, take it off the levelers etc.
We now consider the toad is good (cheap) insurance - even if we don't use it, its good to have.