Everyone travels and lives a little different, so our situation would not be the same as some others, but I'll give it a go:
Our largest expense is fuel. However, we move a lot. We average about $1,000 a month in fuel costs. I work part-time in an office in Nashville, so that means a lot of our trips are round trips. I'm working on changing that and having them let me work remotely more often. That would lessen our fuel costs dramatically.
Our second largest expense is health insurance. It was in 3rd place until we sold our house in Alabama and declared Texas as our home (via the Escapees mail service giving us a Texas address). Our insurer (Golden Rule) doubled our rates (from $500 a month to $991 a month), indicating that Texas requires more coverage than what we had signed up for. Looking ahead on the government insurance web site, we can lower our rates back down to around $650 to $700 a month through a silver level plan through BC/BS of Texas. We have to wait until open enrollment on November 15th for next year's coverage, though.
Camp sites are our third major expense (was 2nd - see insurance, above). Yes, most campgrounds will have weekly (usually 6x the daily rate) and monthly rates. The latter typically don't include electric, which is usually metered and paid for separately. This varies from campground to campground. We budget an average of $30/ night for camping, which comes out to $900 a month. We can typically get by for less than that.
Aside from that, there are groceries, vehicle maintenance and repairs, trailer maintenance and repairs, tags and licenses and mail forwarding costs from our mail service. Most of those are variable and can't be pinned down exactly, but mostly should be estimable on your end.
After posting this and re-reading it, I almost forgot. Our biggest expense this year is actually the co-insurance and deductibles we paid on top of our health insurance premiums. My wife had gall bladder surgery early in the year and I had an appendix removed in May. Our plan has a $5,000 per person deductible, then pays 70% above that. Between our two hospital visits, we ended up paying around $11,000 more in medical expenses. This would come in the "emergency fund" budget column. As it is, I have the flexibility to work more hours if I want, so I did that this year to pretty much offset those costs.