How early is early when going to an Aire

We are going to France very soon in our MH, we've been before with caravan, but have no experience of Aires. While we intend staying mainly on sites, there may be the odd time we use an Aire, the advice so far has been arrive faily early or don't leave it too late but what time of day would that be.

The other thing is we will have a small car on tow, how will that impact on using Aires is it a no no or is it allowed but frowned upon?

We are heading for the Loire and the Dordogne, but will keep it fairly loose as lots of posts on CT have encouraged us to be a bit more adventurous, so we could change plans if the weather isn't good there.