We full-time on less than 7K per month, including insurance and taxes.
We have a fixed budget, about $2500/month, which includes taxes, health and rig insurance, phone, satellite, rig mortgage, etc.
We have a fluid budget, about $3500/month: food, campgrounds, fuel, maintenance, and entertainment.
We have a savings budget, about $500 per month. This goes towards major expenses if they should arise: major medical, major repair, etc.
The fluid budget changes but we have a general outline for spending. When one category gets high for the month, we cut back on another. For example, gas was very expensive through Canada on our way to and from Alaska, but we made up for it with boondocking and cooking at home. In Florida this winter, our campground costs will be high but we will use less fuel.
So far, it has worked for us, even after our first year victory lap (seems everyone has one) and our second year trip to Alaska. Starting into our third year, we are finally slowing down which will cut down a lot on our fuel; the current gas prices don't hurt either. Since we prefer state and federal parks, our campground fees won't go down. We have months where we go over budget (Alaska) but we also have months we are under budget; over the course of a year, it all balances out.
Before we went full time, we fully expected spending a lot less than we do. Campground prices have gone up consistently but gas prices have gone down. Food varies by region as does dining out but both have been rising. Since we have some measure of control on our expenses in all these categories, it is pretty easy to control the budget. We eat lunch out instead of dinner. We get movies from redbox instead of going to the theatre. We spend time in smaller towns rather than big cities and take day trips into high priced cities.
We expect to finally 'settle in' now that the newness is wearing off and we no longer feel compelled to 'see everything.' That is the hardest part of the first year and why I think many people fail; it is so new and novel, it is treated like an endless vacation. From experience I can say that a never ending vacation is exhausting, both mentally and physically (and exhausting on the budget, too). But, allow yourself the victory lap - save up extra for it. It will allow you to learn your likes and dislikes on the road and to prepare for the next few years of full timing.
We have two years under our belt and it is has gone entirely too fast. Occasionally we discuss when we will stop but the idea always depresses us, so we stop discussing it. We are 49 now and currently expect to full time until we are bored. We don't anticipate getting bored for the next few years.
Advice: Enjoy and don't over plan. Be flexible, open, and positive. Know how to use duct tape to repair anything. Carry lots of duct tape.