D3 is the official RV lot but it costs big money to park there. The vast majority of buses & rvs (and people who tailgate) park in G2/G3 these lots are pay when you get there.
You will want to get off Hwy 30 at the Duff exit and head north. Turn west on S. 16th Street (Kmart is on that street) and then see map above. They open the lots 6 hours before the games (normally people are there well before that, including me) if you arrive before that time the line starts on the south side of the street at the spot marked above and will stretch up to the vetmed lot (marked in green). For morning games most people arrive the night before and camp/tailgate all night in the vetmed lot and start lining up early in the morning.
Let me know if you have any more questions. This is my 5th year tailgating with the bus there.