Ian,
First things first. Please don't worry. Whilst I appreciate that very few of us 'enjoy' the unusual, from my experience of French roads and traffic signs there is very little to get in a lather about.
I note that you are heading for Le Mans.
There is a peage(toll) road from Rouen to Le Mans and that is the A28 south of Rouen. If you want to do that then the signs all the way from Abbeyville, through Rouen will be obvious and very easy to follow. My latest information is that it costs 10 Euros solo and 18 Euros with a caravan.
If you don't want to pay that toll then follow my directions (above) and go to Evreux. After the above directions you will be on the N154 which is a dual carriageway. After bypassing Evreux you will, after about 17mls arrive at a roundabout near the town of Nonancourt. From that roundabout you have the choice of right (N12) to Alencon and then D338 to Le Mans or left to Dreux and then right onto D928 via Nogent Rotrou to Le Mans. This choice of routes is not exclusive. There are loads of very good quality roads in the area and I have noticed several alternatives whilst looking at the map in writing this thread to you.
That is all part of the enjoyment in visiting France. There are literally hundreds of villages and towns that we, the British, have never discovered, that would be, if in the UK, be described as tourist 'honey pots'.