Loading anything (Truck, Motor home, Circuit breaker) to 80 percent gives you some "Headroom" as it were, Thus it is a good idea to so limit.
Is it necessary? Not that is a whole different question.. I have seen things that are easily capable of dealing with 2 or 3 times their "Rating".. (No, I will not open myself to liability by naming them)
The one exception is your Motor Home maximum water pressure.. The max pressure sticker is already at 80%... (Though the PEX lines and the fittings can easily take 100 PSI or more... The seals on at least one common valve used in many motor homes is only rated to a max of 60 PSI new, if older that goes down a bit, so 60*.8=48 PSI,, 45-50 is,,, Right about there.