You are correct, brake fluid is not petroleum like most everything in the world. DOT3 and 4 fluids are made from a glycol-ether … Dot 5 from something else.
Synthetic suggest it is synthesized, but like synthetic oil, it is not. Not sure what the synthetic denotation means, but it doesnt really matter.
Fluid will absorb some water. If the brake fluid exceeds 212F then that water can turn to steam, and when you release the brake pedal and try again that steam might have pushed a lot of fluid into the resivoir, displaced by steam, and when you go to push the brakes this steam easily compresses and then there is not enough pressure to push the actual brake pads to slow down…
More likely they will get spongy and feel soft, but still stop the vehicle.
Seems like changing would be prudent, but I defer to old mechanics that laugh at this even for RVs.
Use low gear. For diesels exhaust brakes are great (for any manual shift trans exhaust brakes will work fine).
In conclusion, someone needs to keep money flowing to the mechanics