You can buy Maxxis ST20575R15 LRD for $86.99 per tire at this discount tire dealer:
http://simpletire.com/maxxis-205-75r15-tl15703000-tires
doing the math, total cost for what is usually agreed upon to be the best ST tire on the market is $467.90 delivered to your door. Then, just find someone to mount/balance them (probably $20 a tire) for a total cost of about $540 for the "Cadillac" of ST tires.
Do some research on here regarding ST tires and you'll find that most are made in China and are called "China bombs" for a reason. Goodyear Marathon ST tires are also currently made in China, so their reliability is questionable as well. It really doesn't matter if Goodyear stands behind them and will replace them when one throws a tread at 65MPH and destroys the side of your camper on the first day of your vacation. Getting a new tire at the next town may or may not get you back in vacation mode. Chances are, you'll be limping home, upset that your trailer is torn up and your vacation is ruined. I'd stick with ST tires from what appears to be the best manufacturer (Maxxis) and the price is essentially the same as the "bombs"
(EDIT): After reading the post following mine, I realized the 279 is a larger Springdale model and uses ST225 75R 15 tires. The ones on the RV are 8 ply, and as stated in the following post, upgrading to 10 ply (LR E) if the wheels are capable of the increased pressure would be a good option. Either way, whether you remain with 8 ply tires or upgrade to 10 ply one, Maxxis would remain my choice for ST tires. Here are links for them, but you may find them cheaper locally.
10 ply: $553 total price:
http://simpletire.com/maxxis-225-75r15-tl15713000-tires
8 ply: $497 total price:
http://simpletire.com/maxxis-225-75r15-tl15710000-tires
I apologize for thinking the 279 was one of the smaller Springdale fifth wheels.