Planning a road trip from kansas to california and back?

I'm travelling from wichita ks through the lower states down I-40 to san Diego with plans to stop at four corners then the grand canyon then into cali then returning home with a planned route through las vegas, salt lake city UT, yellowstone park in WY then through denver back to kansas. I'm curious as to about how much it'd cost to make the trip plus have extra cash for misc fun/sightseeing, motel fees, toll roads, and for emergencies. I've travelled a little but never a trip this long. I've rough estimated my vehicle at around 15-20 mpg or a little more on the highway given the driving/weather conditions, driving speed, and road conditions like slopes etc. Rough estimated the trip to be around 4,000 miles. Just want to make sure I'm gonna have enough to take the trip and have fun along the way.