This is going to vary so much from person-to-person. It all depends on how you live and budget in your stick-house life and how you live and budget in your full-timing life.
We've been on the road for eight months.
We lived in one of the most expensive places to live in the country, with a mortgage to match (even for our small townhome). When we decided to full-time, we fully financed a brand new truck (0% APR!) and also partially financed the Airstream. So we consider our the total of our car+truck+campground payments our new "mortgage", and though the new "mortgage" is less than the old, it's not even close to 50% less. It's stil quite a savings, however. I know a lot of people travel in fully-paid-off TVs and trailers, so there's a huge savings there for them! Also, we tend to boondock less and stay in more full-ammenity-type parks — mostly for the electric and Wifi I need for my job (I do web design and development), so others could theoretically spend A LOT less on campgrounds than we do.
Gas: If we were to compare what we spend on gas now to what we spent, say, three years ago, when both of us had long commutes, we spend less now. The truck guzzles it more, but even though we're exploring the entire country, we're putting in far less miles than when we both commuted to work every day. About three years ago, I started working from home and we bought Prius—less commute and better mileage. Now we spend more on gas than we did in that phase of our life.
Food: We spend more on this on the road. Factors:
•*No more Costco — no room to carry stuff that big!
• When you're in a new place all of the time, you don't always know where the good food and values are, so you tend to just buy food where you can, and can't plan the shopping as well.
• I'm a foodie, so every time we hit a new spot I find about ten restaurants that I just have to try. At home, we enjoyed eating out semi-regularly, but here, all the new shiny restaurants at every turn are just too tempting. We eat out a lot.
Communitcations/Internet: Because of my work and the necessity to have reliable internet, I have two 3G cards from different carriers, which totals about $120/mo —*more than internet at home. But we don't have cable, a land line, etc. But, we do have XM Radio, OnStar, Netflix Streaming and Hulu plus, so it's probably near a wash.
We have the same cell phone plans we did before
No electric bill. We've spent an average of about $40/mo on propane since we've started, but it's also been cold for about 80% of time we've been on the road, and we really prefer the furnace to the electric methods of heating, so we used it a lot, and we expect that to go way down for the next several months.
Miscellaneous shopping: has gone way down! There just isn't room to just get "stuff" you want without having to get rid of stuff you have, so it makes purchasing decisions a lot more critical.