Fuel, vehicle maint and such is going to cost close to 50-65 cents per mile.
By working at campground selection - most folks can average $20 - $25 per night for full hookup sites, unless you over age 62 and willing to use Corps of Engineers and other federal parks. That can drop the average down below $17 - if you are willing to use sites without sewer hookups.
Where you plan to travel makes a big difference. In the central US there are thousands of COE sites at hundreds of campgrounds. In the northeast - fewer federal sites, many are US Forest Service without hookups.
Out as far as the Rockies and west coast - the number of federal campgrounds goes down, but the number of federal boondocking sites goes up. The 5er size, weight and truck will limit the number of federal campgrounds you can visit.
Many commercial RV parks in Colorado and New Mexico has better rates if you are willing to stay a month, or a week. Always talk to the CG before you arrive and make certain you know the rate. Compare two or three whenever possible.
If you are talking $3,500 per month - it's doable. You can't stay a high end resort RV parks often, but that's not a bad budget.