I have doors on the RV and the toad with locks on them, before opening the door to the vehicles I go to the pump and look at the numbers, then I let the attendant lift the nozzle that I want and proceed to unlock and take the cap off, some time ago one idiot tried to switch caps on me, by placing the cap on top of the pump and taking the one in his pocket, he did not see me retrieve the cap, big surprise when the cap he had did not fit and I placed mine in the filler.
The other thing that I always do is have the correct amount of money ready and I let them know how much to pump, I have the money in my hand and I hand them one bill at a time and count the amount.
The other time that they tried to scam me was a gas station in Los Angeles CA, the pump had 0000.10 when I reset it, went in to tell the attendant, he really got mad and told me to get out, I lodged a complaint with Weights and Measures, the gas station got a huge fine and was closed for 90 days, scams in Mexico are common so be aware and complaining does not work, they are going to tell you not to be dumb.
navegator