Three month road trip across the USA; what do I need?

My dream getaway is to take a lengthy road trip from Washington, D.C. (where I permanently reside) to San Francisco, to Miami, and back. I've already spent very long in thoroughly tracking routes to take, completely excluding major Interstates. I have prepared an itinerary that lasts nearly three months with many cities, towns, and sites.



I plan on taking either a car, or an RV, something to be comfortable in. I'm not sure if I should bring along other people, but it would help with gas prices and any tolls I may come along. I'm trying to leave space for spontaneous stops and random places to explore, so I haven't planned much beyond that. I also have the problem of financing it. I think it would cost so much money, even though I plan on sleeping either in the car, on a campground, in a tent, or in an RV, I do plan on staying overnight at certain destinations (Vegas, L.A., Dallas, Miami, Myrtle Beach, places where people would stay at a resort) to sleep on a bed and just have accommodations indoors for a couple of nights.



I imagine my entire trip to cost at least 8,000USD (excluding the cost of the RV) for supplies for the road, medical supplies, insurance, indoor stays, resorts and parks where there is a charge, pit stops, reserves for medical emergencies, outdoor gear, and entertainment.



What else should I be prepared for on the road or at destinations? I'm planning on seeing everything between DC and L.A. so you know that it includes a hefty lot. I want to be prepared. I also have a cat. Should I bring him with me if I have an RV? I don't trust anyone else to take care of him for months while I'm away, plus I want his company.Three month road trip across the USA; what do I need?
PLENTY of money, clothing (duh), camera, plenty of film or memory cards (if you have a digital), and of course all your daily needs (medicine, toiletries, etc), and definitely bring along some food in your RV (obvious)