WELCOME JON!
I get roughly 18 with the Predator in 65hp mode and MBRP exhaust, if I drive normal - BUT I have a VERY hard time keeping my foot out of it. If I put the rig in the 100hp mode it sucks fuel like you couldn't image.....but it's amazing to drive.
I doubt you'll be disappointed with the the truck you mentioned - I LOVE mine!!!!!
I've seen polls where quite a few people claim to get 20 to 21 driving normal, not towing and not getting on it.