I have a 10 year old KZ Frontier 28 foot travel trailer that I've had for 7 years now and in the last few years have had a blowout (or 2) on my annual long distance 800 mile trip to Florida from Texas...
I doubt you will be able to move to the 16 inch wheel , you should be able to find a better tire in the 15 inch size . Choice between LT and ST is yours alone to make . I prefer tires rated for a higher speed and weight capacity , running a tire at or close to its maximum capacities both speed and weight can lead to problems .