A good friend of mine formerly owned a Hi-Lo. It was one of the dumbest ideas I ever saw. It was utterly useless while on the road. It was impossible to stop and run back to the trailer to make a quick sandwich for lunch or even to use the restroom and even when the upper body was raised all the furniture suffered from the compromise necessary to fit under the lowered top section plus it had all the heavy and complicated electrics/hydraulics for the raising and lowering. He finally was forced to all but give it away when he grew tired of it.
About ten years ago five of us couples with four trailer rigs got together and traveled to Red River, NM from Lubbock. We all stopped and gassed up on the north side of Lubbock and because the women wanted to stop again in Clovis, NM, a distance of 100 miles, we topped off our tanks again. I was driving a '94 Ford F-250HD w/fuel injected 460 V8 and E4OD transmission and 4. 10 gear pulling a 9,500 lb. 34' triple axle Airstream. My good friend was driving a '93 Chevy 1/2 ton Suburban w/fuel injected 350 V8 and TH-700R4 automatic w/3. 73 gear pullling a 7000 lb. 24' Hi-Lo with half the frontal area of my Airstream. We ran together at the same speed, same road, same wind, etc. We both filled our tanks side-by-side and calculated our fuel mileage. I don't remember now what the actual mileage was but he got 1/2 mpg better than I did. So much for the advantages of a Hi-Lo. I don't think they are made any longer.