There is no such thing as synthetic oils. Synthetic motor oil is just higher quality petroleum. Syn brake fluid is just a higher number. It all comes from oil. The higher number fluids usually can take higher temps, so maybe the "synthetic" is a better choice for a heavy vehicle, but not all brake systems are meant for just any fluid you pick out.
You can never change brake fluid if you prefer. As vehicles started to get better and last longer and need less maintenance then we started seeing things like "change brake fluid" or "rotate light bulbs" .
Yes, some water particles might get in the brake fluid and have an effect, but you can run the life of the vehicle and never change the fluid, it will still work. Do change your brake pads and do use high quality pads for a motorhome. Cheap pads are the wrong choice.