We actually just left Collier Memorial State Park in Chiloquin, where we stayed for 2 weeks in full hook up site, and used it as base for area, which included 3 day trips to Crater Lake (50 minutes drive to rim.) (Made those reservations 6 months in advance.) While there, we also drove by Diamond Lake RV park, and thought it would be good, closer alternative. Some considerations:
In June BOTH parks, and whole Klamath Basin area, is overrun with mosquitoes! Collier sprays maybe 1x per week, which knocks them down locally for couple days. Locals advised that once mosquitoes go, black flies move in, so...hey, your camping, come prepared! Our screen house was welcome respite at Collier.
At moment, east side road around Crater Lake still closed due to snow, as are most trails...unless you are snowshoeing. Limits your activities then. But, 11 mile trail around Diamond Lake is gorgeous, open, and good for biking...bring DEET!
There is are no decent grocery stores near Collier, so restocking is a 35 or more minute drive to Klamath Falls. Situation probably worse at Diamond Lake.
Number of other interesting things to do near Collier, like the adjacent logging museum, and incredible Train Mountain Museum site. Check it out on campground reviews.com.