Hi David!
The on-line check-in is a bit different at the
Fort Wilderness Campground Resort. When you enter the campground they will still ask you to go through the lanes for check-in but because you have done the on-line part of it this will only take a few minutes. They will give you your site location and if it is not ready from the night before they will direct you to the overflow area. There are two sections and you are free to choose the one you feel comfortable with getting in and out.
Secure your motor home and make sure everything is locked. You are not allowed to keep generators running so if you have an animal you will need to make arrangements for it so it will not be left inside while you are away. You will have rights to all of the Fort Wilderness amenities and Disney transportation when you arrive even if it is early in the morning. You can walk to the main bus depot after parking and head to the parks and/or Disney Springs, however, it sounds like you will hit the parks the next day. Make sure your magic bands are linked and head for some fun. If you would rather stay in the resort you can hop on the internal bus and stop off at the swimming pool for a day in the sun. You will also find the Bike Barn which is where you can rent fishing gear, canoes, kayaks, and more.
You can also visit the Pioneer Hall area where the Disney stables are located. This is not where you book the horseback riding but this is where all the beautiful horses you see in the parks and parades call home. If you have little ones they can even enjoy a pony ride there.
You can certainly spend a day of fun at Fort Wilderness no matter what time you arrive on your check-in day. You just likely will have to wait for your site to open up but they are quite efficient getting the sites prepared as quickly as possible.