For our type of travel we are planning on $36,000 a year after taxes (in present dollars). Earned income is taxed in retirement. I'm no tax expert but what I understand is if you did not contribute the money that is in your retirement plan, earn income while retired from paying jobs or take from an IRA/401K then it's taxed by at least the federal government. States vary on this.
Our planned style of travel includes earning $6,000 a year or working for a camping spot to meet a budget deficit. We plan to make use of monthly camping discounts. The $6,000 is part of the $36,000 a year budget.
I used the budgets from two other longtime fulltimers to come up with our budget. However, one of the major unknowns is what health insurance is going to cost.
I am also under the impression Medicare does not kick in until age 65. At least that's the case for me (I'm 52 now) and my wife who is 60 years of age.
We are not retiring until 2019 so I added some inflation to our budget. We are also keeping some of our money to buy another house or ungrade our RV if we stay on the road longterm (more than 6 years). I did not budget for the RV's depreciation which is a cost if you think about it.
You may also be aware social security benefits are reduced if your earned income is higher than around $15,000 each year. I'd have to check on what the current limit is.