$3500 is not even going to cover your fuel costs during the year. Figure on 10 MPG with diesel at $4 a gallon or $0.40 a mile. $3500 will cover your fuel costs for 8700 miles of travel.
Campground fees vary from $7.50 a night for state and federal parks for seniors to $75 a night at many private RV parks. Having a full size truck towing a 35 ft. trailer is going to limit where you can stay and add to the cost per night.
You food costs will be same as they are now so if you are spending $400 a month on groceries at home you can expect to pay at least that much on the road.
Internet access on the road is going to be 3-4 times the cost of what you are probably paying at home.