I would do the U/S. I don't think there is any way around it for you.
We have a similar (but way milder) problem here. Ellie had a UTI and was treated with antibiotics. She kept urinating in the wrong places (on furniture in particular which she does not do). We took her back to the vet. They couldn't get a urine sample (small bladder), so the vet did an xray to rule out stones. No stones so more antibiotics. The problem cleared up for the most part but she did still have an accident on occasion. This seemed to intensify and one day she had to pee three time in less than 3.5 hours (not like her at all). I talked to the vet and they did another u/a and culture. Everything is fine. Ellie was spayed and the problem came back or stayed (one of the two). The vet just said to get pH strips and bring her in if it s 8 or above because we can't keep running there.
Those are on order now and I really think we should do an ultrasound but they are so expensive. The vet is thinking more behavioral or possibly diabetes insipidus in this case.
Anyway, it isn't constant and she never really leaks. She peed on the bed two days ago and on the couch two weeks ago but other than that she is perfectly normal. I dunno what to do.
Yorkieluv, I have a question that maybe you can answer. When the vet spayed her she said she looked at her bladder and everything seems fine. If the vet felt her bladder, could she have felt ammonium urate stones? I know I need to ask her this but I'm not due to talk to her for awhile...