When I test another regulator it behaves as I expect it. For my tests I'm just wiring Input and Ground to a 12V battery and probing the output pin. Using this setup the old regulator is giving 5V out and the new one I'm trying to use is giving 12.
Any ideas what might be wrong other than a defective Reg?
Micrel makes some of the very best LDO regulators, but you need to use them correctly.
Your -300 device need to be loaded with a minimum of 7mA.
Either add a resistor of maximum 680 Ohm, or use a red LED and a 330 Ohm resistor over the output (ground to out) and you should get the nominal 5V output.
To keep it from oscillating, you need 10µF on the output as well, but that is not your imminent problem. Likewise, if you have more than an inch of wire from battery to input terminal, you should have a cap on the input as well, but that is for keeping the regulator from misbehaving as well. It will still test good without the caps, as long as you apply the minimum load.