Can you use plastic at the Mexican stations to avoid exchanging money? Would the attendant not want US dollars for a tip? Very ignorant about Mexico here.
Most Pemex stations do not accept credit cards. If you find one that does, you'll need to first make sure your card issuer knows you will be in Mexico -- sometimes they stop the transaction for fraud prevention otherwise. Also, most cards charge a fee on transactions in a foreign currency, so know before you start what you will be charged. Lastly, some transaction processors have an upper limit on how much can go on one slip. Once, we put over 3,000 pesos in, and the card kept coming back "declined" until the attendant figured this out. Once he broke the transaction up into two parts of less than 2,000 pesos each, it went through fine.
If the station accepts US dollars (as many border stations do), then you're OK to tip in dollars, too. I suspect they'd rather get US$2 than MX$20 right now, anyway. Further in, though, you should use pesos, as the hassle factor for them to have to change a small number of dollars is probably high for the attendants.
If I wanted to have a diesel storage tank, does it have to be something special or can I use a large water tank?
I can't speak to Mexican law. Here in the US, fuel must be carried in a DOT-approved container. Water tanks are not permitted.
What are the customs limitations on bringing fuel back? Must it be in a tank connected to the vehicle fuel system or can you put a big independent tank in a pickup bed?
They will not question anything in your vehicle fuel tank. If you have jerry cans or other tanks not built in to the vehicle for motor vehicle fuel, you must declare it. It is then subject to the $800 personal exemption.
Has anyone used the crossing at Mexicali?
Yes. Search on that term in my blog.
-Sean
http://OurOdyssey.BlogSpot.com