If you can charge
at your parents' place (a dryer outlet would do fine) then you should be good to go in that respect. Maybe wait until the Seabrook, NH supercharger is done so you can top up on the way, since 270 miles one way is pushing it. Or buy the CHAdeMO adapter and patronize one of the stations in Portland. CHAdeMO isn't as fast as a Supercharger, but it's fast enough to be a practical bridge for road trips.
Lack of charging at home is a big problem for sure. I would not buy a Model S if I couldn't charge it where I lived. Many apartment building owners are amenable to installing chargers, but many aren't.
Note that you don't necessarily need a garage, just dedicated parking somewhere reasonably close to electrical infrastructure. Houses with at least carports or driveways are pretty common places to live.
"Never take road trips further than 250 miles" needs a strong amendment. "Never take road trips further than 250 miles where charging can't be done at the destination and you're outside the supercharger network and away from CHAdeMO stations." Sure, it happens, but it's pretty rare. If you're staying within the Supercharger network, or you stay within one-way range and can plug in at your destination, then trips over 250 miles are no problem.