First, I don't travel with a trailer.
Second, sometimes you can talk to the person running the campground about leaving your bicycle in a storage shed or some place like that. When I cycled in Australia for 3 months in 2004, that's what we did when we wanted to go somewhere for the day.
As for your stuff ... well, everyone else in the campground leaves their tents up with their gear in there when they go out here and there for the day.
Some campgrounds are more secure than others. Some I've gone into and instantly felt uncomfortable ... I've moved on. Others leave a much better impression.