there are only 2 entry points into Bulgaria from the north, east side is via a bridge over the Danube at a place called Ruce, and west side is on a car ferry at Vidin. The east route is probably better for you, because you will get to see more of Romania this way.
On the east route there is a campsite between the border and Veliko Tarnovo, (a town with loads of brits) which is run by an english guy. It's called Trinity Rocks Farm, and he runs rock climbing trips from there. This route takes you south on good roads, passing through the Shipka pass, which in winter is often blocked by snow. From there you run down to the town of Stara Zagora, which is a good town.
The west route takes you through good mountain country on good roads, or via smaller not so good roads, you can choose. This route you go to the capital, Sofia, and then head east mright across the country, to the turkish border. This journey will take about 4 hours from Sofia, if you don't stop, but I don't know if there is anywhere to camp on this route.
If you are camping come and see us, we're about 30 kilometres before the turkish border. (Home
If I can be any more help you can get me on email@example.com
Martin T. Jeffes.
Sakar Hills Touring Park