What is the Best Way to Pay for Purchases in a Foreign Country?

It is possible to change money here and there and everywhere, but to compare apples to apples, the FACTS can be found at this link.
Credit Card, ATM, and Debit Card Charges
These are the charges that are applied for Credit Card, ATM, and Debit Card purchases.

Credit Card, Traveler's Checks, Cash?

I have found the best exchange rate for "foreign currency" is using an ATM. I generally spend summers in Canada and check rates from ATM against Credit Card regularly via internet. The ATM is always a better exchange rate.

Merchants do not charge to exchange Travelers checks, but unless you buy them in US Dollars in Canada you are not likely to get the best rate. If you do exchange in Canada you may do a little better but still won't get the best rate.

If you plan to be in a foreign country for an extended visit, it is easy  to determine which way is best for you. I use the ATM to get cash while in Canada. On the same day I will make a Credit Card purchase. When I get home I log on to my Bank and see how much the ATM cash cost me in US $$. Then I log on to my credit card to see how much I am billed for my CC purchase. ATM, as I said earlier is always a better exchange rate.

I don't know if the same principle applies when you are in the US, but you can surely find out within a day or two of arriving.

How much is enough for a year of fulltiming?

Do you want to know  how much you will need for a year of fulltiming? A rough  figure would be from $25,000 to $40,000.  Few spend less, and many spend more, much more. It depends entirely on your lifestyle. How much do you spend now? Don't forget to include insurance  - medical as well as the rig and if you fulltime in it you will need fulltimers insurance. If you drive a minimum 10,000 miles at 10 Mpg that is 1000 gallons. Just for the heck of it you best figure on $3.00 gal, or $3,000 for fuel. If you manage cheap camping at an  average $300.00 month that is $3600.00 for the year. Do you have to pay for any medication? Add that cost. Cellphone if you use one, Satellite TV if you want it. Internet. Repairs and maintenance.

We have been living this way (fulltiming) for most of the last 30 years and would not swap it for any other lifestyle at this time.  I think the most important thing is having enough reserve funds to comfortably handle any emergency and be able to continue with a smile