Christi, I think anyone would need a bit more info in order to answer your question -- how many windows
and how big are they. Essentially though, you need to decide how full you want them -- 1 1/2 times or 2 times the width. You also need to know how wide your fabric is. Since you asked in yards (not metres) I will assume that your fabric width is in inches -- likely 36 or 45 inches? So, if your window is 3ft wide by 2 ft deep (just as an example) and your fabric is 45 inches wide, and you want double fullness then to figure out the number of panels you need you would take double the 36 inches = 72 inches. Divide that by the width of the fabric (45 inches) and you can see that you will need a little less than 2 full widths of fabric. So, if your length is the 2 feet, and you want to allow some extra for hems -- assume an extra 6 inches -- then you will need 2 1/2 feet x 2 of fabric. That will leave some left over on the side unless you wanted your curtains really, really full. Hope that makes sense. So, the yardage depends on how many panels you need for each window x the length of the window (plus allowance for hems, rod pockets etc.)
BTW, have you considered mini-blinds. We have them on our trailer and really like them, they pull up right out of the way and are really need looking.