There are no hills you cannot get up pulling a loaded trailer if you have low enough gears. A friend went up Fargo Street in LA pulling his son in a Burley cart. 33% grade and the sidewalks were stairs. He had a frankenbike with a 3 gear/inch low. Consider a triple bike with a 22 tooth front and a 34 tooth rear. If you still have trouble you can spring for a Schlumpf Mountain Drive and wind up with something like an 8" low. Harris Lake Cyclrey has them.