In the Loveland/Fort Collins area, the Big Thompson is probably out of the question for the reasons the other posters have stated.
There is the Cache la Poudre River, known by the locals here as the Poudre (pooder) that has fly fishing spots. There's parts of that river that, I've been told, are trophy areas. Jax Outdoor Gear in north Fort Collins could give you a LOT of information about fishing the Poudre.
There's also a KOA campground with camping cabins right at the mouth of the Poudre Canyon, at the intersection of US287/CO14. Here's a link that might be useful if you choose to consider that area:
https://koa.com/campgrounds/fort-collins-poudre-canyon/site-type/cabin-accommodations/
2007 GMC Sierra SLE 3500HD Dually
2016 Coachmen Catalina Legacy Edition 243RBS
2007 Keystone Outback 25RSS - R.I.P.