At Flyng J's we've encountered water flow at a trickle at the RV lanes and I would not describe the water as potable. The ones on I75south below Atlanta in particular...and the cramped conditions and line of people behind protesting taking too long. Idon't remember if there was a connections for hoses or not or the hose just ahd an end to let you get a trickle into your radiator type thing, certainly not intended to flush your tanks if emptying. A federal campground on the lake not too far from us has a dump station for anyone for 5 dollars and potable water hose connection..however flow was slow.
I would bite the bullet and fill up my tanks in an overnight stay before you get to south Georgia and Florida, where most of the water has a lot of iron and sulfur, other minerals, farming chemicals and will clog a lot of filters. Use a really good filter....and maybe a water softrner.