RMS level from impulse response
Please refer to Parseval's theorem.
If your impulse response is $h(n)$ and its DFT is $H(k)$, then
$$\sum_{n=0}^{N-1} |h(n)|^2 = \frac{1}{N} \sum_{k=0}^{N-1} |H(k)|^2 $$
So the square root of the sum of the amplitudes in the frequency domain, divided by the number of points in the DFT should give you the RMS. It is common practice to calculate RMS on a frequency band basis to get more meaningful results.