Buy the trailer park. Sell it to the tenants. Their rent payments become mortgage payments, in effect. When they finish buying the property en masse, it becomes controlled by a tenant-run cooperative.
This is, obviously, a money-losing proposition for whoever buys the trailer park in the first place. That's why it should probably be a government entity.
If the trailer park residents can do it by themselves -- and in several locations, they have -- that's great. But it would be preferable to have self-owned communities over slums, even if you have to have the government provide the initial loan and organization.
Is it socialism? Sure. The good kind.