Even though the boat has electric tilt, is there a way to manually lower it into the water so I can start it?
It is a Johnson 90 hp 4 stroke.

There is also a mechanical lock.... meaning a bar that is between the motor and the transom that is in the "travel on road position" .
It is a rod about 1/4 inch in thickness and has a metal ear on the outside of the bracketry... you'll need to lift the engine very slightly after locating that ear and then flip that mechanism down out of the way. Then your motor should be able to be lowered properly.
A good idea is to also buy an adjustable bracket to support the lower end and prop as you travel down the highway. They come with bungee chords and are also highly adjustable to prevent cracking your transom when you hit a pothole or bump. Good luck. Also should have tie down belts that keep the hull of your boat to the trailer as you hit bumps in the road.