We have been fulltiming Since January. I budgeted $750 per month ($25/day) for campgrounds. So far, we have gone over that twice, once by $15 and once by $35.
We have paid from $0 to $48 for a night stay. We have not stayed at free locations (rest stop, walmart) more than 4 nights in a month. We have not stayed at a private campground for more than 6 days so have not gotten weekly or monthly rates.
We use Passport America if it is convenient (and it has often been very convenient) to pay 1/2 price at campgrounds. We stay often in Federal, State, and COE parks. We have also stayed at Escapee Parks - they are often a great bargain.
The two months we have gone over budget were both in Florida during the winter months. Snowbirds seem to lock up the state parks early and most private parks don't offer PA rates. But, if you are willing to move around, even within the same park, you can often put together enough time to see the area. The trick is to not be afraid of moving down the road. There is also value in looking at the reservation site every day - we got many of our sites because someone else cancelled.
We budget $1000 a month for fuel. We go over that when moving a lot. We are way under it when we don't (We were over in May when we went all the way across the country and in July when we came back east).
This coming summer, when we head to Alaska, our fuel bill should go way up. But our campground bill should go way down as will spend more time boondocking. So it all evens out. If we think we are blowing our budget, we cut back on eating out (which we do a lot) to bring the total number back in line. Keep your budget flexible and you won't have to sweat it so much.
Good luck! The first few months of full timing are the hardest. Getting used to being in an RV but not being on vacation is the largest adjustment. Once you get past that, its all easy.