Yeah I rode on 28c too, if you could go bigger then I would just as some of the route can be a bit bumpy with tree roots sticking through the tow path, but 28c tyres were easily enough.
As for the route, here's the strava track from the campsite at Creon (amazing site with a pool and lovely restaurant) down to the super U in town and then on to the McDonalds/Super U at Eysines, once you're at Eysines it's all very straight forward, just follow the cycle path to the sea! If you look at the rides either side of that on my profile hopefully you'll get a decent idea of the rest of the route, linking from the canal to the Piste Cyclable Roger Lapabie can be tricky but from what I remember it was signposted okay. You can orger a map off a French website too, google voie vert or route entre les deux mers and that shoild find it I think.
As far as campsites go, the Camping Municipal at Valence D'Agen was great, an easy days ride frok Toulouse, when you get into town on the canal come up onto the road and turn left away from town, follow the road and go straight over the roundabout and the campsite is next to the horse stables on the right, the campsite on the left of the road was pretty dodgy so I'd avoid that one ha ha.
Another good campsite was at La Reole, just before you cross the massive red suspension bridge down a little road to the right, it was basic but did a job.
Best campsite of the lot was definitely at Creon, Camping Bel Air (I think) as I said great restaurant and bar and swimming pool. It's a little bit up the road (uphill with your back to the town) past the car garage as you cross the road on the cycle path in Creon.
Once you get past Bordeaux too the sea there are plenty of campsites on the coast, I stopped at the last one you come to before the sea, they were pretty expensive and this one was the cheapest but most basic, had everything you need though and lovely little wooded pitches.
Hope all that info helped!! Sorry for the wall of text, it was a couple of years ago so I suppose routes/repairs might have happened to different bits of the route but to sum up, it's easy to navigate, lovely countryside and an abundance of cheap well appointed campsites.