While I'd take CarringB's advice over my own, here are some _additional_ thoughts:
Simple/dumb thing to check is always tire pressure. That and working my engine oil too long/hard has resulted in similar drops for me. A couple 1k changes fixed the latter, but really it was nasty sludgy looking oil that drove me to that.
(Well, and towing a heavy or non-aero-dynamic trailer, but you'd be down on power if you somehow managed to not notice that in any other way
Obviously, 4-low on trails burns fuel at a truly impressive rate.
(my experience is on a 99 v10 on 33.5" tires)