Here is my workflow with RX:
1. Find a section of your recording where there is nothing but room tone. This is critically important. No clapping, talking, nothing. Zoom in close and look at the spectrogram really closely to check there are no intermittent background noises. Select this area. It doesn't have to be very long, maybe around a second or so. Too short and RX won't have enough information to build an accurate noise profile.
2. Open the Denoise module and change to the following settings (this is what I have found through lots of trial and error works best for HVAC noise, but your situation may be different):
Spectral Tab
Manual
Reduction Curve (I found this helpful in that the processing more or less leaves alone the treble range where its effects are more audible, but try it with and without.)
Quality D (processing will take much longer but the results are much better)
Artifact Control 7.0
Threshold 0.0
Reduction 6.0 (the default is 12.0 but I find that causes audible artifacts with the HF range; 6.0 removes most of the background noise but appears to not have the negative effects. YMMV.)
You might also try the Rumble Reduction preset and tweak settings from there. That's where I learned to try the Reduction Curve setting, as this preset enables that be default.
3. Click Learn while your noise profile is selected. If your selection is too short it will tell you.
4. Still with the Denoise module open, move the cursor to a section of your recording and click Preview in the Denoise module. Listen to see if it's doing what you want. I keep toggling Bypass to make sure there is nothing undesired happening. It's also very useful to Preview while selecting Output Noise Only. When that happens, you should only hear your background noise. If you hear periodic blips or other phasey sounds, then you have a noise profile that's not good and should start over.
5. Once you're satisfied, make sure Output Noise Only is unchecked. Then Ctrl+A to select your entire file (or whatever you want to denoise) and click Process. On my computer it takes about an hour to process a 1 1/2 hour recording with these settings.
6. Listen to the results to check it's how you like it, and then File > Export to save it as a new file. I always add "RX" or "Denoised" to the file name so I know that has been applied, and I record the exact settings used in a text file.