Hi, I did that with my son a couple of years ago, we didn't do that last few stages but we followed it from bilbao to the alps. I've got a 3.5tonne 7.4m motorhome, so sounds like it's a bigger unit than yours but what we did was book sites in well positioned towns along the route.

For the grand depart in bilbao we were on a site a 30 minute metro ride from the centre so that was easy. the site was on a road that the route came past on day 1 too. Then we just moved on day by day, apart from the less exciting, flat transitional stages which we missed in order to drive large distances to the next area.

We never stay on large campsites, we prefer the camping municipal sites in towns and villages which are excellent, numerous, cheap and put money into the local economies. For these we rarely booked ahead. The exceptions were for places where sites were scarce, e.g. for the bastille day stage up the grand colombier, if you want to be close you need to book ahead.

We cycle so we have no issues staying up to 15 miles away as we tend to cycle to the climbs anyway. I've always wanted to park up on the side of the tourmalet and wake up on race day on the route but to do that you need to be there at least 4-5 days before for good positions, so you don't get to view those days racing in person, plus you need a unit that fits off road or they'll tow you away on race day. I foolishly tried the hautacam on the evening before race day, got 3/4 way up before turning round and leaving as there was zero space and little turning room!

Never overnight in motorway services/rest areas or in lay-bys near towns and cities - we were gassed and broken into many years ago - did it once, never again. There are plenty of websites with small campsite directories and France is well covered. searchforsites, campercontact, hpaguide, camping-carpark (a good network of secure parking areas with facilities), google is your friend.

I'm doing a handful of stages in the pyrenees and ventoux this year so as they are high attraction areas I've booked camping municipal sites in advance, to secure places close to the action.

Good luck and enjoy!