A tip: if and when you have decided where to go, go to the French autoroutes site www.autoroutes.fr - you will need to know a little French. You can set your start and finish points, specify if towing, and also specify if you prefer not to use the autoroute. It will quickly work out the best and cheapest route for you. If you have selected non-motorway it will still take you on the free sections and occasionally on short sections of chargeable, but the rates are not high (about 10c/km towing) and as others have said they are quiet, smooth, and good for towing.
Also being a newby, note that Route Nationals (RN) use a green sign, autoroutes are blue, and local road signs are yellow. If you see a blue sign and it does not say 'peage' then it is free, but there is a tendency to tell you only at the last minute. Worry not, there is always an escape road just before the booths (unless the access is directly off a roundabout.)
As for road numbers, A and RN roads should not be a problem, but the French started renumbering their local road system in 2006 and it is still ongoing. Generally a two or three digit D road will be peculiar to the Department (county-ish) so you may find yourself using, for instance, unconnected D18's several times in different areas during your journey. However the D9?? series roads are either former RN's that have been downgraded or rerouted, or they are cross-Department routes. Takes a bit of getting used to but is essentially straightforward.
Lastly remember that in many areas, especially urban, the 'priorite a droite' still applies. This means that a vehicle joining from a side road has priority - even on traffic islands. The worst bit is that locals using such side roads shoot straight out onto the main road more often than not without even a glance to see if something is coming. On traffic islands even where traffic on the island has priority, the French usually go right around the outside of the island, rather than using lane discipline as we do. Net result is that if you follow our practice you may well find someone coming round your outside and cutting across your nose. Watch and understand the road signs and you will have great fun.
(GB on your plate by the way is the only two words in English that every French driver knows - Get By - and they do!!)