How many lugs do your wheels have, 6 or 8? A 6 lug maxes out on a 6K axle, 8 lug is generally on a 7K axle. Whats the listed GVWR on your RV? A 4 leaf sounds kinda small for the weight your listing. A 5 leaf spring usually starts in the 2900# range. Broke one on my toy hauler a couple yrs ago and replaced all 4 springs with the next heavier weight rating. Went from 4 leaf to 5 leaf. Had to replace the broken one along side of the road, bought replacement at a tractor supply type store. Make sure you replace all u-bolts when replacing springs.