Assume the meter is for the lab.
1) you need the polarity of the DC adapter.
2) Eventually, you'll have to be able to determine if a heater is open.
3) You need to size the 12 V supply.
Without TRMS the relationship of P=VI won't be valid at the output of the dimmer, but you can use I of the input and V of the input with a guestimate efficiency factor. The freq response of the TRMS matters too.
Non TRMS meters measure the average of the rectified waveform and multipy it by a number that makes a sine wave input read in RMS. It basically makes P=VI valid for the same AC(RMS) and DC voltage.
I'd also recommend a precision resistor to be determined. e.g. if 1 A is expected, R=0.1 V/1 A. This will allow you to measure power easily. 100 mV is negligible compared to 12 V. P > 0.100/1 A
Also remember we think you need a bigger 12 V supply. You need