You can get a 7 to 5 pin adapter plug to make it simple.
I hate that setup. Your just inducing another plug connection and another place for something to go wrong.
Don't usually see 7 pin plugs on boat trailers. Waste of money when you can get by with the 4 or 5 pin flat plug. Much cheaper. Especially with the corrosion we have to deal with.
I say BS. You will find 7pin connectors on "MY" trailer's that had 4pin connectors. That is the way I rewire them so I don't have to fool with adapters and get a good tight fit into the 7 pin round female on the truck.
It's just one of those little things that makes life easier down the road, every time I plug the trailer light in and disconnect them. No having to go get and plug the adapter in.
I have a triaxle trailer here right now. I can't move it due to a stupid 5 pin connector.