Welcome to iRV2, we're really glad to have you here as a new member and are looking forward to reading your posts about your RVing experiences and adventures. I noticed this was your second post since joining in '03.
You'll find a great bunch of folks on this site who are very
knowledgeable of RVing, the rigs we use, and the places we go in them.
Can't be sure what's dragging your mileage down but often times we add more weight to our rigs as we use them over the years. Is it possible that you're running a little heavier now than before? Might drop it on a J-Scale next time you're at a Flying 'J'. I'm sure you've addressed all the basics such as tire pressure, alignment, easy on the go pedal, air filter, etc.
You might want to post this over on the gas section of the The Diesel Stop
, there's usually some pretty good discussion going on about the V-10.
As you'll see, they're eager to help and share their knowledge and experiences with others.
Again, welcome to iRV2, we're glad you joined.