FRP isn't as sturdy and can delaminate. Once water gets inside the layers the wood paneling (assuming they used wood sheets, which is common in boat construction) can deteriorate quickly.
Steel is sturdier. There are plenty of stories of Wanderlodge owners surviving crashes, hurricanes, and fallen trees that would have seriously damaged or destroyed a lessor motorhome.
With all of that said Wanderlodge was about the last holdout for steel construction and there is a reason for that - steel is hugely heavy, and you get a vehicle with relatively poor fuel mileage (expect about 4.5 mph for a PT series Wanderlodge and maybe 8-9 for a Forward Control). That, and as slideouts became popular the extra weight was just unmanageable.
So you pays your money and you takes your choices. Foretravels have a reputation for being a really nice FRP build but you probably want to look at the individual unit before any money changes hands.
Ditto with Wanderlodges - that steel body usually holds up well but sometimes everything else has been neglected.
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