I seems perfectly clear to me what the OP's question was: "As a rule of thumb", does a motorhome (not a TT or 5'er with an un-powered chassis) type RV chassis age a lot faster - due to the motorhome's weight - than the chassis of a light weight vehicle does (such as a family SUV)? The "rule of thumb" being to mentally double the actual mileage showing on a used RV's odometer when trying to estimate it's life left, as compared to the same actual mileage showing on the odometer of a used family SUV.
There being maybe just a slight bit of truth to this "aging a bit faster" thinking ... as regards to the weight being carried ... is why we bought a Class C built on a chassis too heavily rated for the weight of the coach. We were hoping beyond hope to get a longer (reliable) life out of the chassis regardless of how we maintained it.