Not quite an HET, but I have a bus-conversion with a Detroit Diesel 6V92TA (two fewer cylinders than your HET's 8V92T), it gets 7.5MPG down hill with a tail wind @ 26,001lbs (empty). I think if I put more in there, that number would go up (of course uphill would suffer then).
Get it towed/trailered. Newer transport trucks should be able to trailer/tow it for the same cost as your drive in fuel alone, and it won't take up your time. As CBR mentions above, if it breaks on the road, good luck finding your nearest local Oshkosh Defense service depot.