I have seen a variety of studies for and against the nitrogen filled tires...
I fill mine with a custom blend of 78% nitrogen... based on over 100 years of research and development.
There is a very narrow set of conditions where a high nitrogen inflation protocol might be advantageous, trailer tires ain't one of them IMHO.
Tire quality, age, loading and proper inflation are way more important.
I have priced up a new set of 15" Dextar wheels and Maxxis 8008 tires for my Coleman CM16FBS which will give me a lot more wiggle room in the capacity department AND provide me with a quality tire. Total cost is ~$575, cheap insurance in my opinion. Original wheels are 14" with "C" rated tires. Good "D" rated basically don't exist in the smaller sizes.
FWIW the stock tires, Westlakes, are rolling at close to 90% of capacity with the trailer sitting empty, add my cargo and that goes to 98%.
With the tire upgrades I will be dropping back to somewhere around 65%-70%.