I guess I need to be careful about going to Iowa. In most of the rest of the middle of the country I've found retail LPG dealers quite willing to fill, although wholesale locations may not always have a technician on site. Most of my fills have been at RV parks; for KOA, I think the service may be a franchise requirement. Locally, I have a LPG retailer at a gas station between my house and the campground at the lake.
On the road generally, Flying J travel stops often have a RV lane with dump station and LPG fill.