You don't need an RTOS to know that a system without extraneous background processes isn't doing extraneous background processing. Where precisely do you think think your timings are going? You're not also running a Minecraft server on the nav computer. Using Linux doesn't mean you also need to enable stupid PC things like search indexing or seti@home. Navigation isn't resource constrained. The only resource constrained component in the whole system is the computer vision module and it's going to be on its own processors, and the rate at which it can hand off new output is on far faaaarrrr longer timescales than your interprocess communication latency.

Yes, thata one of the RTOS thing, but almost always, when trying to build this down into what these timing limits are, they are kinda arbitrary, gut feeling put into a number. There are exceptions, yes, usally these exceptions as well are only for a limity subpart of the system. So for this using RTOS for everything is as stupid as not using RTOS, (or possibly even puting these parts into hardware, ASIC of FPGA) for these small subsystems. These timing used, also is mostly up to scheduler and Linux is possible to run with a scheduler that gives it similar capabilities ans most RTOS systems. It's a blury grey area at best.