I believe self-powered behemoths like motorhomes are required by regulation to have marker lights, not just reflectors.
Most likely the stress cracks are a combination of an inferior marker light, combined with an installer armed with a powerful clutchless pneumatic drill with screwdriver bit. Zoom, zoom...
Those lights are less than $5 MSRP, so factories probably buy them for a couple of bucks each. Have to be rather cheaply made for that price.
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The only thing that works on a RV is the owner...
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