I used to have the Tamron 18-250, which was the forerunner of the Tamron 18-270 (each was rebadged as a Pentax). It was pretty decent for a superzoom, but going to a two-zoom kit was a huge improvement, especially at the telephoto end.
If you want to stick with a superzoom, the Sigma 18-300 is really the only option:
Sigma 18-300mm F3.5-6.3 DC Macro HSM (Contemporary) Lens Reviews - Sigma Lenses - Pentax Lens Review Database (I think it has now been discontinued so might be hard to find.)
If you go with a two-zoom kit instead (and if you want real improvement, I'd suggest you do), I'd recommend this lens for telephoto:
HD Pentax-DA 55-300mm F4.5-6.3 ED PLM WR RE Reviews - DA Zoom Lenses - Pentax Lens Reviews & Lens Database It has outstanding autofocus, very good resolution, pleasant rendering and is light-weight, compact and affordable.
For longer telephoto your main options are the DA*300mm f4 with the DA 1.4x teleconverter or the DFA 150-450mm f4.5-5.6, but these are much heavier and more expensive.
For the wide end, the main options are the DA 16-85, DA 18-135 and Sigma 17-70. There is a case for each. To minimise lens changes (and it's very affordable), the 18-135. For best IQ and extra width, the 16-85. For in-between, the Sigma.