My knee-jerk reaction is that you've put too many constraints on your wish list. 1,000-1,500 miles, to my knowledge, just isn't possible in a loop format while staying off roads. You might be able to largely stay off roads on the continental divide trail, but that breaks the loop.
Having said that, the Adventure Cycling routes do an excellent job of keeping you on lightly traveled roads. Their magazine had an article a few years back where a bunch of kids wondered if they could ride a mile naked in Kansas without anybody seeing them. (Spoiler: they chose the wrong mile.) Most TransAm veterans smiled, lots of roads are that empty.
Off the top of my head, you might try Lewis and Clark east on the Katy Trail, then head north on Great Rivers, then west on the Northern Tier until you meet up with L&C, then head back south. (Adjust to fit where in Iowa you want to start, of course.) NT has been re-jiggered to use more rail-trails in Minnesota, and of course you've got a couple hundred miles on the Katy. I suspect that's as close as you can get to your previous wish list.
One more point about the AC maps, they'll have points of interest called out. You might want to ask the various state tourism offices for maps, or get the state books from AAA, and plan out additional near-but-not-on the-route stops you want to make. That should keep you busy for a few months while waiting for summer to arrive!