You can make a splitter for your 7 pin but I would recommend you install a separate connection for the camper. I'll bet you're camper's cord is short and located toward the front. I would install a connection in the truck bed near the camper's existing cord. You can then wire this into the truck's existing wiring.
If you truck happened to come with the fifth wheel prep package you may have wiring for a bed mounted plug tucked up under the truck somewhere.
There's nothing wrong with leaving the camper on the truck all the time. Many people do.