I don't have this mod installed (yet), but have had a browse on the forum posts here, and I would *guess* that the mean time between failures (MTBF) isn't set to be conducive to long duration usage of RCS thrusters as primary propulsion. Perhaps go into the part config files and manually edit them? Previous posts offer advice on where to find the settings. You might want to think about realism, if that's your thing... RCS is pretty reliable IRL for station-keeping and so forth, but I'm not sure about number of engine restarts and so forth. PS - Is this the radial monoprop engines? Or RCS thrusters? If the probe is small enough you can probably get to orbit on RCS thrusters on Minmus, so I'm not sure which part(s) you're talking about. I can imagine the RCS thrusters might have a low MTBF set in the config files, because they don't fire for long durations and otherwise they'd never fail.