There is a bus directly from the campsite to Porte Maillot, where you can take the Metro 1 to the City. It's about 2€ one way, if I remember correctly.
An alternative is the RATP Bus 244 also from Porte Maillot. The bus stop for the camp site is Moulins Camping, the ticket is ~1,10€ one way (depending on your type of ticket), the last bus is around 22.00.
The problem is, that you have to use one ticket for the RATP bus an another one for the metro trip to the city. YOu can save the bus ticket by going to the metro station by foot. It's about 20-25 minutes to Pont de Neuilly on the Metro 1. Half the way you walk through Bois de Boulogne, the other half through the upper class suburb Neuilly (Where Sarkozy was mayor...). There you also find some nice restaurants and cheap supermarkets.
Another way to get to the city is to walk thorugh the bois de boulogne in eastern direction, then through the posh 16th arrondissement to Trocadéro and the Eiffel Tower.
On the other side of the Seine river there's the nice little suburb of Puteaux. There's a main street with shops and some nice boulangeries-patisseries.
From Puteaux you can walk over to La Defense through the back door.
I've never had problems with the reservation, however I've been there in May and September so that may be a difference.
There my old trusted Nokia 3210 was stolen, the first and only time something like that happened to me on InterRail.
The camping site itself is OK, there are enough showers, not too dirty, too too clean; most of the space is occupied by caravans/mobile homes/bungalows.
One area for tents is close to the Seine but unfortunaltely there's a fence; the other area is close to a main street through the bois de boulogne with quite a lot of traffic.