Anna, I had major problems with RSI, as does everyone else including the French. I had an accountant but she wasn't doing the 'right thing' as she was in the process of moving on, as it turned out later. A friend in the same business (immobilier) recommended a firm with offices in Draguignan and St Raphael and although it was a little costly having them wade through 2 years of mess, they sorted everything out including crazy and duplicate demands for different amounts (somehow I'd been given 2 TVA numbers) and got my bills reduced to an almost reasonable amount before helping me to dissolve my sarl.
I'd been told to expect to pay above 25000 E and eventually paid closer to 19000, which was a relief, although I still didn't have a Carte Vitale! That finally came when I registered AE instead.
They also looked over all the demands that came during this year and said they were mistakes that they'd sort for me, even though I no longer use them since the end of the sarl. It's good to have someone on your side, if you can find a good accountant, if only to reduce the stress levels.
My local RSI is about 1.5 hours away, but I discovered there's a weekly travelling office open in Fréjus for 2 hours on a Monday afternoon. You might make enquiries to find if there's one nearer to you? They've been very helpful; as Jan says seeing them in person makes such a difference both for you and for them - being able to put a face to the name makes you a real person and they start working to help you. Having said that, they're just as puzzled as I am at some of the letters I receive! But at least the misunderstandings are registered and hopefully something gets done.