Load range is how much weight a tire can carry. That's determined by how many plies (layers of material) are built into the running tread that touches the pavement. More plies = more weight. Tire makers make tires the same size that can handle differing amount of weight to accomodate different trailer types and axle manufacturers.
The axle of most popups are rated either for 3,000 lbs or 3,500 lbs (if you are single axle which is what limits the weight and cargo capacity of a popup). The weight of the trailer, all the stuff inside, all rides on the axle and two tires. It's all about safety.
Here is a table of info for the tires I have. This is not an endorsement just info: look at the difference in the load range (C or D) and the Max Load Capacity (weight each tire can carry):
https://www.carlislebrandtires.com/our-products/product-detail/radial-trail-hd
For my ST185/80R13, note two Load Range C tires equals approximately 3,000# and two Load Range D tires are approximately 3,500#. So buy the tires your axle can handle but don't (on a combined basis) exceed your axle's weight rating.
So I don't know if you have any paperwork on your popup that gives specs on components or can do any google research on it. Or get a visual on the ground inspection and maybe find a name or serial number. Most popup manufacturers used a only a handful of suppliers.
If you buy the Load Range D but your axle can only handle Load Range C, that's a little extra money wasted and a false sense of security that you can carry an extra 500#. If you get the LRC but can handle the LRD, you're cutting about 500# off because now the tires become the limiting factor on how much weight you can carry (but hey, you saved a few coin).
So my question was do you know your axle rating and what tires are on there now? (size/brand).
Bama Ed
PS - my point was to figure out what the popup can have but you can default to LRC for the rim size you have then it's just brand/seller selection. Sorry if I made it too complicated. Goodyear doesn't even make a trailer tire my size anymore (they did when I had bought a replacement set on my old Jayco about 8 years ago).