One thing I've learned, and others here will attest to this: NEVER 100% trust what a HD dealer does, regardless. When you pick up the bike, check it over, especially if they changed fluids. It would not be the first time I have read about somebody picking their bike up a quart low, overfilled, scratched, something is loose...the list can go on. There are some really good dealers out there, but they seem to be the exception to the rule, and there are some really good techs out there too, but again, the exception. The "techs" that do oil changes are rarely their best people...it's kind of like taking your car to one of those quicky oil change places where they strip the oil pan bolt, don't tighten the filter, etc.
It would be interesting to know what your oil temp was, but it's water under the bridge now. Even if you don't put an in the dash gauge, you might consider a dipstick temp gauge. I have had good luck with the HD LED version, but others have had trouble with them. The other benefit, IMO, to the LED version is you can quickly get a reading of oil level when you stop for gas, etc. Not saying that it will be dead on accurate, but mine have always been really, really close. Other folks think they are junk. There is an analog gauge out there too, but I cannot think of the brand/company that makes it. 110Thunder has one. Not to be constantly monitoring the oil temp, but just for spot checking when you get in unusual situations and just "want to know". It's not something to fret about...the 110's run HOT, regardless. Synthetic oil can take temps well beyond what Dino oil can take before degrading. The dash gauges on a Harley are not the most accurate things in the world, just general indicators of what's happening. But 15psi at 75mph is unusual, and NO pressure at idle on a modern engine would concern me a bit. The old motors often showed no pressure at idle, but the newer versions of the twin cam (since '06 or so) generally show 8-12 on the gauge. If a gauge is suspect, the only way to know is put a reliable gauge in the line and check it against that.