Actually the LT tire or a all position tire will better withstand ply shear stress's long term (side scrubbing around corners/backing/twisting/etc) than any brand of ST tire.
LT tires is nothing new for trailer use , just lots of new folks to trailering. We used light truck tires on our heavier trailers before the tire industry ever gave us the infamous ST tire.
Your '10 Sandpiper may have 6.8k-7k axles and requires a tire with a minimum of 3500 lbs capacity. Sandpipers web shows the new models starting at 15500 GVWR and up.
This means a Sailun S637 235/85-16 G or the Goodyear G614 in a LT235/85-16 G at 3750 lbs and 110 psi is gonna' be your best option.
Both tires are a commercial grade all steel ply carcass. Both are recommended for regional trailer service industry and work great on a RV trailer.
All tires and wheels on our RV trailers have close spaced tires in the center of a long heavy box. Flexing is normal and is the main reason tire experts and experienced tire mfg recommend max sidewall pressures on this type of trailer.
Some may recommend a XPS Rib or a R250 LT E at 3042 lbs. These tires don't have enough capacity for 7k axles.
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