That just means that they are moving all of the time instead of stopping to enjoy an area. We are starting our trek to Mesa, AZ from south of Olympia, WA tomorrow - about 1500 miles and it will take us 5-6 weeks! We get 8.75 mpg (has stayed steady for over 16 years), so less than 200 gallons. Even with high CA prices, that will be less than $800 for the 6 weeks. This summer, we have moved every 2 weeks, (less than 50 miles for most moves) on one tank of fuel (100 gallons) or less than $100/month.
The budget of $3500 is what I would consider reasonable. Not over extravagant, but you would be comfortable while you are traveling. We were able to take early retirement because our retiree benefits included FULL medical coverage at no cost to us until we turned 65 and that plan is now our Medicare supplement so the only thing we ever pay for is $20/90 supply of any medication. We couldn't have had such a comfortable retirement without it.