Remember, because of the Homeland Security issues, most states require a physical address within the state to allow a driver's license and registration in that state. They might also require a certain length of time staying within the state.
I guess if you really want this discount you should look into changing your domicile to Pa. Each state has it's own rules on how to do this. My wife and I did pay the NYS sales tax for our coach, all $17400 of it, then after we both were retired, moved to and changed our domicile to Florida. There was an exact procedure for doing so including a physical address (we initially used a mail forwarding service for this), driver's license, registering to vote, registering the vehicles and declaring our domicile to Florida.
I suspect the requirements will be similar in Pa. You will have to decide if the time and effort are worth the savings.
FMCA #F431170, GS #822128658, Escapees SKP #112655
2012 Airstream Mercedes Interstate Class B