I am sorry you are having so much trouble. I know how discouraging illness in the flock is. I have been there too.
It's not necessarily that Rhode Island Reds are not a healthy breed. But that many hatchery birds in general are not the healthiest of birds over all. These birds are over bred, they can have genetic issues that continue on down through many generations, they can have weaker immune systems from being over bred, the conditions in some hatcheries may not be up to par.
Not that all hatcheries sell poor or sick birds. But you do take your chances from hatchery stock. Generally your feed store will order their chicks from the nearest hatchery or from the hatchery in their state. So generally, all feed stores in your county will get their chicks from the same hatchery.
Heritage bred birds are going to be the most healthy of birds. They are not over bred and are usually bred in very controlled environments. Sandhill Preservation Hatchery is a classic example of good stock and healthy birds.
Even mutts that breed in chicken yards, hatched under broodies can be fairly healthy. They are hatched on your grounds, from birds that have developed immunity to your area and are raised with a broody.
I have always gotten my birds from feed stores. I have definitely had my share of the health issues in my flock. Each breed had their own set of problems over the years. But I have enjoyed them and they live a good life at my place.
Your best defense is a good offense when it comes to chickens. Keep your facilities and clean as possible. Give your birds as MUCH room as you can afford and more. I am a big fan of probiotics. 70% of immune system is in the gut. This is where many pathogens take hold as well. So if you keep good oxygenated bacteria in the gut and the immune system beefed up, you have less health problems in your birds.
Get them outside in all weather and keep them wormed. Very important! Worms devastate a flock, killing them over time.
Keep them on a good diet, put out several feeding and watering stations so there is no competition for food or water (higher ranking birds are known to run off the lower in the order birds, thus weakening their systems) and watch for bullying in the flock. Bullies wear down the lower ranking birds which causes all kinds of health issues.
Keep in mind, not all chicks do make it to adulthood, but most of them should. If you are seeing a high rate of deaths in chicks, either the strain has issues, or something is a miss in your facilities.
If you continue to see deaths in your flock, it never hurts to have a necropsy from a vet done on a freshly dead bird to see just what is running through your flock. Immediately refrigerate (not freeze) a freshly dead bird in a plastic bag while you make an appointment with your vet.
And speaking of vets, it never hurts to take a bird in to one if they are knowledgeable on chickens or avian medicine. Most are not avian specialists, but some are.
As for the one that is limping, inspect the feet pads, feel for heat in the pad or leg and on up to the hip. She could have bumblefoot developing in the pad, she could have sprained a muscle or pulled a tendon somewhere in the leg.
Keep us posted! :-)