Hey, I went to both last year on a UK bike, UK passport, UK carnet, and found them to be two of the easiest borders in Africa. Lovely people in both countries. Two of the coolest countries in Africa I thought.
The carnet isn't essential as far as I am aware in Rwanda,we certainly didn't get asked for it. Nobody seemed to know at the Burundi border, but the guys just filled it out as I asked. They didn't have a clue on entry or exit. I just did it to be safe.
In Rwanda camping is difficult, I agree. If it exists, it is expensive - With the exception of the 'Car Wash' Campsite in Kigali. Amazing place, run by a top Kenyan guy with A1 hosting instincts. It's not really a campsite, but they have a lot of space, top top top notch food, and they run a good bar there. I think our friends put it on 'Tracks for Africa.' Highly recommended. No charge either, they just hope you'll buy lots of food and drink I think!
If you are struggling for money, some of the big banks in Kigali will let you withdraw money on your card from behind the counter. Saves expensive Western Union kind of transactions.
Hope this is of some help.