Hi lacers!

to the DIS Camping Board
As your order of steps above indicates, you need to probably leave the campsite (official checkout time 11am) before you head to the parks. You have two options in that case.
First, you can go up to the front of the Fort on the end of the parking lot furthest away from the checkin building which is the "overflow" parking lot for extra flat bed trailers, storage trailers, and early/late daily arrivers (like yourself). The earlier in the day you go, the easier it is to find a spot. Buses to the non-MK parks leave from this area.
Second, you can drive your RV/toad to the park of the day and the Parking CM's will direct you to a place to park for oversized rigs. You get a parking pass for your dashboard when you arrive at the Fort which is good until the end of your check-out day so you won't have to pay the daily parking rate. Might be the best choice for all but MK.
Driving to the park-of-the-day probably is best for all but MK. For MK, the bus/boat option to MK leaves from the Settlement end of the Fort down by the lake so the overflow lot might be the option. You COULD drive to the MK parking lot but that would mean driving out of the Fort, turning right, then going down World Drive, turning left in front of the TTC and Poly, then left in front of the golf course, and out past the Speedway gas station/WDW Car Care Center and out World Drive then make a U-turn on the INSIDE/left 180 lane to head back thru the MK Toll booth (no cost to park because of dash pass) and park in the MK lot like an off-site visitor). That's a LONG way to go.
So you can certainly do what you want to do but the decision depends on which park you want to spend that last day in.
Bama Ed