If there's no event going on, you can park for free near the stadiums and catch the subway from there. It's not immediately clear exactly where to go when one gets there; asking around is perhaps the best idea, or making reference to the map from the web site after getting one's bearings. Even if you have to pay to park, this might be one of the more convenient and reasonable options...though I don't know what their rates are on non-free days.
I would not want to try to navigate Philly, nor especially try to find parking in the city proper with a trailer. It's tricky enough enough with a small car. Many streets are quite narrow, and corners sharp, and a lot of the parking is in multistory garages that are not at all suitable for trailers.
XFINITY Live parking info