We take the train all the time from Philly to Orlando. I also take it to Greensboro, NC to visit family. It is expensive, especially for a room, the earlier you book the cheaper it will be. They let you book 11 months in advance and I try to get it as soon as possible, once a room is taken, the price goes up for the remaining rooms. They never seem to offer discounts on rooms either and their rewards card isn't worth a thing for any type of room discount. And every year, the prices go up and up.
My fiance's family is in Missouri and we have looked into trains that direction - would be impossible - too many connections and too long a trip. Chicago is the main hub in that area so all trains have to go up to Chicago - there aren't really any trains moving east/west. Anything going to Florida looks like it has to go from the east coast - doesn't appear to be any other lines going straight to Florida.
The only way to really ride cheap on Amtrak is coach but I would not recommend that. The bathrooms get super nasty. I have done business class a few times, which is not offered on trains with sleeper rooms, and the bathroom still got nasty after a 10 hour trip. I can't imagine doing a 20 hour trip. The last 2 times I tried business class, I got dehydrated from not drinking so I didn't have to use the bathroom - pathetic, maybe, but the bathroom was Nasty!
In Feb 2013, we were on the train with a couple from Ohio, they got off in Washington DC and then caught a megabus back to Ohio. The megabus seems like a good option but they don't seem to have a ton of lines right now either. Maybe something to look into?
Would 2 roomettes be cheaper for your family than 1 family bedroom? You might want to look into that option. One time we were on the train with a big family from NY and they had a bunch of roomettes for everyone.
Good luck and happy traveling!