Please take a look at the UPP package here. This package describes in detail how to post process model outputs and calculates brightness temperature. UPP is not specifically developed for MPAS, but many methods used in UPP to obtain diagnostic variables can be applied to process MPAS data.
Apologize for the delayed response.
I would assume that the variables are the same for each RTM no matter if you use RTTOV or CRTM (or any other models I am not aware of...)
CRTM can be downloaded here: Index of /jcsda/CRTM/REL-2.3.0