Boyd Lake and Riverview are the 2 that I would recommend. Boyd Lake is practically in my back yard (I live near the lake to the west of Boyd), and is the nicest place around the area. As was pointed out, there's common water in the loops, electric at the sites, and showers as well. It's very popular, so book as early as you can.
Riverview has been completely rebuilt. It was destroyed in the Big Thompson Flood of 2013, and took them about a year to rebuild. The interior roads are dirt/gravel, and there are sites that border the river as well as more interior sites. The new landscaping is starting to mature nicely, and it's pretty much bounced back from the devastation of the flood.
These are really the only 2 that I would recommend in and around Loveland. There are 3 KOA's around Fort Collins (one just outside of town on the north end/one in Laporte at the Poudre Canyon turn off/one north of Wellington). They tend to have a goodly number of long term sites at all 3. Others in Loveland area I would not recommend for more than an overnight stay.