Here’s its symbols.com entry; nothing really jumps out at me, though. There are also various related entries here.
As a WAG, perhaps it’s related to the pentagram in its meaning as a ward against druden? Those were often painted on doorsteps to protect against their nightly visits.
I think their origin involves heavier cast iron plates that were sometimes put on the outside of brick walls to anchor reinforcements. One 5 story brick building I know well was moved about 100 feet and got this reinforcement treatment. The plates were sometimes plain squares but also sometimes given decorative forms. I suspect the new, lightweight, nonstructural decorations that have been popular in recent years may take their design from those older plates.
Well, at our State House they have a bunch of those on the walls and in the floor where Sherman’s cannonballs hit. Probably that’s not what they mean on your mobile home, though.