do fluids do any harm when left in too long?

all that would be prevented by the next service!

The answer is it depends and on far too many things for me to list.

OIL for example Blow by on the piston rings can cause the oil to turn acidic over many miles.. Now oil has acid neutralizers in it so this does not eat a hole in your oil pan...> But if you drove far enough .. Well I've actually seen it happen one time on a car.

Brake fluid (some of them) can absorb moisture out of the air then the brakes fail I've never seen that myself.. but i've read about it.

next year before we use it we would get everything done.
but the question was has any harm been done?
thanks

You should have the fluids changed, and have a qualified chassis mechanic check bumper to bumper. 2016 is 7 years ago and 2019 is 4. How old are your tires? You need to have them inspected if not replaced as soon as possible. Probably need to have the roof inspected and resealed.