I flew a twin to Tucson and parked at the Jet Center FBO for two nights and 3 days. I did a bit of flying around the local area the second day and took on fuel from them (at not a vastly inflated rate). The aeroplane was looked after, they had free food and toilets with mouthwash in

and were very friendly. When I came to leave I asked the lady if there was a ramp change...No charge she said
Palm Springs is another example, flew in there, they lent me a car for the day (FOC), I took on fuel, and when I came back, no charge.
In fact I don't think I have ever paid a "ramp fee" at any FBO. Santa Monica had a $200 fee for a Citation, but taking on fuel and it was waived.
I remember Carlsbad airport had a $3 over night fee in Transient parking, which was an honestly thing. You put it in an envelope and stick it in a post box. Almost all the other ones I have been to have been free.Essentially most US airports are free to the end user. I did once phone one up and ask if I could land (as in UK PPR). The guy on the phone was baffled..."of course you can land, this is an airport"