You don't need to use the original bushing, just use the metal sleeve from the originals it fits right into the Hellwigs bushings. this way you can use the stock Ford bolts with no clunk. That is what I did with mine. 2500 miles no problems.
on our 2008 35 foot MH we get 7. Uphill, downhill, towing, loaded, unloaded, air running, generator running it always seems to average out right at 7 MPG.
15,000 on ours. Will probably replace them next year due to age. Leaning towards going with the same tires but not 100% sure yet. I am happy with the way they ride and they are wearing okay.