What would cause both rear tires to go on a tri axle? Just went about 700 mi. Both tires were toast by the time it was over, tread gone, belt showing etc... Most of weight seemed to be on front two axles?

Crooked axle and/or bent spindles.

got your old cig home?

is the camber out of the axel, worn out shackles, bent spindels, , fell off the ramp on a short launch????

old tires,,what is the mfr date,,their should be a set of numbers in a outlined oval lookin bow..more that 5 years in the sun will plat hell on tires..also you say most of thge weight was on the front 2 axles,if the trailer were to sway then the rear tires would take the most abuse,espically if the trailer was on a forward angle..

most of the time, old tires have belt seperation

They were the oldest of the bunch. I will check the axle. Thanks