How long has the RV been sitting? Rust on the rotors can make brakes not as effective. Also the brake pads or rotors could be glazed from heavy, hot, braking. If you are buying from a dealer, ask that the brakes be bled and new fluid put in. Have a mechanic inspect the vehicle for you, or test drive it to a Ford dealer that will work on or inspect it for you. If it's got a Ford V-10, it's on a Ford F-53 chassis. An inspection will cost less than having to repair later.
Can't really answer the stopping distance question, as has been said it won't stop like a sports car, but it should react much better than you report. Stopping distance would also be affected by how much gear and liquids you have aboard.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI