Hi there, nbka1bm,
DH and I are in similar straights. We got a 61 Safari last fall and found that the old cushions were truly nasty in appearance...and smell! So, we got new foam cut (considerably more expensive than we expected) over the winter and I am now sewing cushions. I was fortunate--I found some appropriately turquoise duck cloth (a medium canvas) on sale for $2.88/yard at the local fabric shop. Without knowing precisely how much fabric I'd need, I took all they had (20 yards) and ended up with plenty! OTOH, my recent purchase of $9.98/yard drapery cloth was much pricier, even though I got much less. I've also picked up a couple of random old drapes in the same colors at a flea market for making toss pillows.
BTW--I was in IKEA the other day and saw nice duck cloth and drapery fabric in a number of patterns & colors for @ $5.99/yard--though apparently you can't phone order it.
I think your 1966
is new enough to have slipcovers with zippers. That's good, since you can tear apart the covers and use them as a pattern (also good for working out yardage requirements). My older unit (made during those quality-concious Wally years) was merely folded and stapled down to plywood bottoms--so they could not be cleaned (and they were nasty). I suppose Wally wasn't too concerned about girly things like cushions.
Mark was right that you would have to compensate for pattern in measuring up for fabric. However, think if the pattern position on the cushion is a major concern to you. Except for centering, say, a big pattern element in the center of a cushion, I'm not sure there are many circumstances with cushions where this would matter. The critical element is that you want the fabric to line up straight and perpendicular on the cushion.
You can also save fabric by using a cheaper fabric on the bottom panels. I have also seen clever fabric economies using fancier fabric panels on the sides or just tops. Take into consideration that all pieces will require @1'" (1/2" on each side) seam allowances, except the zipper panel, which will be split lengthwise for the zipper insertion--that takes a 2" side to side seam allowance. I would also suggest that you get a long zipper that wraps around the edge--it makes inserting the cushion much easier. When toting up your fabric requirement, add in one length of fabric (@2.5 yards in my case) for a fudge factor--there's nothing worse than ending up short (especially when you make a mistake)--and it is well worth the extra expense!
Before you rip out the cushions, take them down to your "foamerie" (ie, foam cutter) so they can reproduce the dimensions. This is especially important for the front gaucho cushions which are curved to fit the trailer walls. Save the plywood--but instead of stapling it on, insert it inside the cushion with the foam. Then zip it closed (and spray it down with waterproofing spray).
I am also applying welting (or cording) the upper edge of the two cushions that show when the gaucho is folded up in a couch. This is harder to do, but it does look sharp. I am also considering "over stuffing" (wrapping the foam with batting to get a loftier effect)--something my girlfriend who does upholstery professionally says is de riguer now. I just wonder if it will make for a less comfortable sleep area! It does have the benefit of smoothing over any imperfections and looks really nice.
BTW--sewing such large cushions is a really, really big job--and with tight corners and welting, rather advanced. I find it takes me a week after work to cut and sew each cushion (or a whole Saturday)--and I'm a pretty experienced sewer. Getting a nice snug fit will also take a bit of resewing as well. If you aren't confident and can afford it, I'd say hire it out!