On a French SIM card supplied by Syma Mobile (wiki), I was being charged with incomprehensible data:

• roaming was disabled !
• and later I even disabled mobile data !

Still I was billed data cost.

And I was not alone. This is why: Phantom Roaming

Learn more reading other various links:

• Phantom Roaming
• Ghost Roaming Explained
• Re: Syma preleve deux fois 20€ sans motif
• Beware of the Fido Roam Trap
• How to stop android using roaming data when it's disabled?
• Turning data roaming off does not work
• Urlaubs-Ärgernis: Geister-Roaming bei Simply (Telefonica-Netz) - Holiday annoyance: Ghost roaming with Simply (Telefonica network)

What I did to reliably avoid such cost while avoiding Plane Mode (so I keep the possibility to receive/answer calls and sms abroad):

1. Disable roaming for the SIM you don't want to be charged roaming abroad.
2. Force the SIM to use a no data protocol to establish connection with antennas: 2G (GSM) or 3G (WCDMA).
   

April 2025: It's now more than a year that I avoid being charged ghost roaming thanks to this technic.

Road trip around France.

The car is a Citroën C4 Picasso:

It is planned to take all these during the trip:

• 1 matelas 140x190x8
• 1 couettes une personne + 1 duvet + 1 traversin
• 1 bouteille de gaz Primagaz Twiny 6kg
  
• 1 détendeur Clip-On bleu lagon (Un détendeur est fourni avec la première Twiny achetée. Source)
  
• 1 tuyau de raccord gaz 1.5m
  
• 1 réchaud gaz Base Camp Lid
  
• 1 carton dans lequel placer le réchaud pour couper le vent
• 1 poêle + 1 casserole + 1 couvercle
• 1 planche à découper
• 1 cuillère en bois
• 2 torchons
• 2 tupperwares
• 2 sets de couverts (assiette + fourchette + couteau + cuillère)
• 2 tasses
• 1 éponge
• 1 paic citron
• 2 jerricans eau 10L
• 2 bac de manutention l.45 x P.35 x H.26.6 cm
• 2 dalles d'agencement 100 x 60 x 1.8 cm
• 2 tréteaux
• 1 poutre
• 1 speaker bluetooth
• 1 chargeur USB / emetteur FM alimenté allume cigare
• 2 batterie portable 10000mAh
• 1 routeur 4G
• 1 savon de Marseille

One application has been used to find places to sleep : https://park4night.com/

An improvement that could be useful is to design a parcel shelf (plage-arrière) in wood in order to put the gaz cooker on it and be able to cook at the back of the car during rainy days. We thankfully didn't get any rainy days except the very first one of the trip.
It could be nice to take two folding chairs to sit anywhere you stop. But I don't know where to store it while sleeping...