Hi Scunnered
We moved here from France about five years ago and brought our German Shepherd with us in the back of the four wheel drive.
We had our pet passport with us and also a letter of 'good health' from the Vet in France, however, we were never asked to produce any documents, not even at the border crossing into Bulgaria (although I must admit the guy checking our passports looked scared to death of her!).
We travelled from France, through Italy, overnight Ferry from Bari in Italy to Igoumenitsa in Greece. From Greece up to Bulgaria and crossed in Sandanski.
We had an overnight in France with a lovely dog-friendly hotel. On the Ferry, we had an option to take a 'dog friendly' cabin, which means she stayed with us for the overnight trip to Greece.
We got to Bulgaria in one day and found another pet friendly hotel in Sandanski.
Basically, we had no problems finding hotels that would take her, nor did we have any problems at border crossings. The dog was by our side the whole journey and arrived quite calm in Bulgaria (more than I can say for me after that drive ha ha!).
It may be worth planning your route and booking dog friendly hotels in advance so you don't have the worry of finding them each day when you've had enough driving.
Hope all goes well for you.
Chris