KTAS (true airspeed) is very dependent on altitude and temperature - it will be higher than indicated at higher altitudes (about 2% for every 1000 feet) and it will be higher than indicated airspeed (IAS) at warmer temperatures (and vice-versa for both). This is also ignoring max mach and equivalent airspeed (EAS) limitations and conversions.
I’m curious as to which temperature and altitudes you’ve tried cruising and what your results were? On a stream I did a few weeks ago, I was indicating 235 but getting about 300KTAS (true) in the King Air at 16,000’, which is right in the expected ballpark. The 50-knot headwind component resulted in a groundspeed of 250 knots. If I had gone up to the service ceiling of 35,000’, I would have expected a true airspeed closer to what’s published in the menu (as those are best-case/max cruise numbers).