I think. If you are going to do that, you should make it a mixture of compost, mulch (woody "brown" stuff?) an green stuff -- see the compost forum sticky. Hopefully, by the time tomato roots reach this layer, they will have broken down sufficiently.
Otherwise, when the roots hit all the browns, they will go into nitrogen deprivation shock, and you'll wonder what happened to your happy tomato plants.
Another way to stretch the potting medium is to mix evenly with about 1/4 sand and composted shredded mulch. This will aid in creating well draining soil and add bulk and weight to help prevent tipping over.
Note, though that most people who have grown tomatoes in containers say 5 gallon is not enough, especially with bigger indeterminate varieties. I decided to try growing tomatoes in containers as an experiment this year, and have chosen containers with 10 gallon capacity when filled to the top. I am growing two varieties that are generally described as 3-5 ft tall at maturity and recommended for/"can be grown in" containers -- Black Krim and Earl of Edgecombe. What variety tomatoes are you growing?