Dependent on where you are going
Fernando,
The cost of being on a roadtrip is entirely dependent upon three factors: 1) where you are going 2) how much you wish to go each day and 3) how much money you want to spend. There is now real way of answering your query without knowing what your range of plans might be. If you stay in government parks cost could be as low as $0.00 and as high as $35. Commerical parks are generally $20 to $50 but usually exclude truck campers. Vehicle maintenance costs are dependent upon how many miles you roam each day.
All that said, if you were planning on driving 300 miles each day, camping in national forest campgrounds and preparing your own food a reasonable budget could be in the range of $25 to $35 per day.
Mark