Here is how bad Forest River is with installing a marginal tire on a trailer. My trailer is 7100lbs dry and has an 11,200lb GVWR. they installed China bomb "D" rated tires. yes a "D" rated tire on a trailer with an 11k lb GVWR. they failed after a year and a half. when I replaced them I installed Carlisle "E" rated tires. the first set went 5 years and 6k miles, I replaced them again with another set of Carlisles and they are now 2 years old. and the trailer handles way better with the stiffer tires. it used to walk all over behind the trailer due to the weak sidewalls.
The wear you are showing certainly looks like its just a tire issue, but if you can check the alignment, its well worth it. then I would step up to the 15" tires and whatever rating your trailer needs and go up to the next rating.
Good luck!