Maxxis are still an ST tire. All ST tires are built to meet a marginal non passenger carrying standard. There are numerous photos of ST tires failing/self destructing. You don't often see these types of failures with an LT tires. LT tires are rated to carry people ST's are not.
ST tires maybe adequate for most trailers but RV trailers and the associated weights and aerodynamics are too much for the ST tire construction standard.
If at all possible upgrade to an LT tire.
IF only ST's are available get the largest payload/PSI rating available and cross your fingers.LOL