Sleepy,
The question is not as simple as you may have hoped.
I have a 45yo motorhome that is aluminum and fiberglass (well, SMC).
My sister and BIL have a 43yo all most all aluminum class C and only the steel parts of the cab are giving them trouble.
A nearby friend has a 44yo all fiberglass coach and has very little trouble.
These are all well built coaches. None have wood framing.
By the same token, a person I know well is about to scrap a 15yo laminate sided coach because of delamination and water leakage that has rotted the wood framing. Whether the skin is aluminum or plastic is of little consequence if leaks are not dealt with immediately and properly.
Matt