Hmm. Good question. The two lake vacation destinations that I am most familiar with are Lake Rathbun (southern Iowa). I grew up about 15 minutes from this lake and spent a lot of time on there boating and camping growing up. There's campsites all around the lake and you can rent RVs (if you are serious, my dad rents his 5th wheel out. Nice camper, just don't use it anymore and is probably available. He'd haul it down and set it up I'm sure. It's Iowa). Good fishing all around the lake and the marina rents out Pontoons/Jet Skis. There is also Honey Creek Resort that has a hotel with a waterpark and cabins available to rent. I'll say, I've had a ton of good times on this lake. If you're looking for views like you have in Colorado, this isn't the spot lol, but it's a good family destination that is cheap.
The other is Okoboji. Its' in NW Iowa and I have less experience here. This is a party lake. Houses are built on it along with bars and restaurants. Alot more money has been dumped into Okoboji than Rathbun in terms of development. It is Iowa's "Clearest" lake or so they claim. Rathbun is bigger, but no houses. Okoboji is also home to the shittest strip club I've ever been too. The dance stage and private dance room are separated by a curtain lol.
You can PM with any questions if you are actually interested lol.