Typically, when there's a suspicion of autoimmunity, a PCP would order CBC, ESR, CRP, RF, ANA, and possibly the HLA-B27 genetic test. Given you've already had negative results on the RF and ANA, I'd specifically ask for the HLA-B27 test. I'd also ask for a referral to a rheumatologist, but don't be surprised if it takes you six months or more to get in to see one. Have you tried a continuous course of NSAIDs to see if it reduces your symptoms? By that I mean, have you taken an NSAID (Aleve, Motrin, Celebrex, etc.) continuously for at least two weeks to see if your symptoms improve? If you haven't and you can take NSAIDs, you should try it, as that will usually provide doctors a clue as to whether your symptoms are inflammatory or not. (Note, I'm not a doctor or medical professional; just a professional patient speaking from my own experience.)
You are the correct age and gender for autoimmune issues to become more likely; so neither you nor your doctors should dismiss your symptoms as "all in your head."