Just wanted to add that Sweet Tomatoes is amazing. You can get lunch for around $5-$6/person...dinner is like $7-$8. There are countless salad options (much larger than even, Ruby Tuesday)...prepared salads as well as build-your-owns with everything from different lettuces to assorted toppings that you could never imagine putting on a salad

(and standard fare).
There is usually also 6 types of soup including a chili - baked potatoes and all the toppings - foccacia, cornbread, and blueberry muffins - assorted pastas (prepared fresh) - and a dessert bar.
It's all-you-can-eat...and it's VERY good quality (I'm kind of a foodie and we really love several of their items - better than most restaurants - the soups, especially, are GREAT!).
As for onsite - we are looking into the ABC Commissary at MGM (which looks REALLY good, interesting, and the prices are quite reasonable). There is countless counter service at most of the parks, we plan to pick up a coffee and bagel or pastry in the morning to save money, then stop for a salad or veggie burger or wrap in the afternoon (most kids meals are cheap - but so are the adult meals! You can typically get a veggie burger combo - or regular if you eat ground beef - for $7...we share a drink *always* so this will be the same)
I found the vouchers to *not* be as cheap as the counter service, unless, I suppose, if you split one meal. Otherwise, the counter service fare (assuming you don't want dessert) is cheaper by itself. We also do things like order a large fry in one combo and just get a burger a la carte to cut back on our fat consumption. This would also be less expensive.
Many of the restaurants in the park have inexpensive ($6-$9) appetizers - you could always just get apps - some are very substantial (my SIL and I do this at Fridays all the time - because they're soooo tasty, but we can't eat a meal after one).
I'm really looking forward to Epcot - they seem to have such a vast array of really tasty treats.
We, too, opted for an extra day - and are cutting back on our food budget - but I think we can still eat in the parks without loading up on junk all the time, yk?
Hope you have a great time!!!