Its going to be very difficult to find land like that, and then super expensive to buy it. Montgomery County... forget it. Half an acre in the county is a couple hundred thousand dollars to start with, finding 5+ acres is borderline impossible. Having it zoned for a trailer is extremely unlikely. Howard County is getting quite difficult too. As people flee the Liberalism and cost of Montgomery County, they are taking out Howard, Frederick, parts of Anne Arundel, etc.
Southern PG, northern Charles or western Calvert County are more likely to have some affordable rural type acreage. You'd need to find a property that is zoned for mobile/manufactured homes. Depending upon where you want to be in DC, its not terribly far away.
