The only road toll you will find is in the North and you must pay online to use a tunnel that spans the Hvalfjörður fjord. It will cost you 1.300 ISK per vehicle to use the tunnel.
The fuel price changes a lot so it’s better to check out the current prices and do your own calculations based on current fuel prices: GSMbensin.is
When this was written, June 14th, 2020, the fuel price was:
200 ISK per liter / 5.6 USD per gallon, on average.
If you take a mid-sized camper, like the Easy Fun, and do a full circle (including stops and detours), you’ll drive somewhere between 1350 and 1500 Km or approx 890 Miles.
As we do the calculation, we use the most expensive fuel stations, suppose 1500Km, and highest consumption and get something around:
25.000 ISK / 186 USD / 165 EUR in driving costs.
Want to do the math with the current fuel prices?
0.08 (fuel consumption per km) * 1500 (km driven) * x (Diesel fuel prices in Iceland)