I don't think there is a law in MD that states you can't own a trailer in another state and bring it through MD for less than 30 days.. it's probably the same as any other vehicle.. So how would the state know you've had it in md more than 30 days without observing you on at least 2 occasions.
Maine is popular because 1> it's cheaper than MD 2> you don't need a title, just a pic of the vin and bill of sale as i understand it.
I usually get vermont when i have no title because it's cheaper to go VT to VA permanent plate than it is to get maine tags if i recall correctly.
Otherwise i get VA permanent trailer tags because paying for it 1 time is cheaper than maine in the long term.
The other thing is, we know MD is stealing and wasting our tax money, so I try to feed the beast as little as possible. At least in VA I get a permanent tag for my money.. Also we get to skip MD state inspection hassles and costs. Also the libertarian in me finds it offensive that a group of people require you to pay them and show them your property before you're able to freely travel not endangering anyone with your own property. If a vehicle is dangerous then the police should be issuing tickets on the road. We should not have to submit to the gestapo to enjoy our freedom.