So, I roll up north for Boyne and have a blowout at 55mph within miles of my destination. I have 225/75-15 E load tires that have the MFG date of 13. I run them at 70psi and they are rated for 80. Looking at them I have at least one more tire that has started to separate/bulge on the tread. I have been down this road before, once one goes it is a matter of miles before serial blowouts begin. Is 3 years all I can expect out of trailer tires? This is kinda rediculous.

I also had the spare blow out, that was understandable considering it was date coded 05...lucky I had a second spare with me.

How do you store the boat/trailer when not in use? Also, what manufacturer are the tires?

You can expect much longer use out of tires on your trailer.

I didnt really pay attention to the brand, I will take a look. The boat and trailer are in indoor heated storage. I did not block it up. This is the first winter I have owned the boat, history before that is unknown except it sat in Shogrens show room for at least 4 or 5 months.

You are storing it right, that's for certain, but likely the original stint was what weakened them. They were likely stored on grass or something else that allows the moisture to wick into only contact portions of the tire, enabling separation. That, and not being protected from the sun/UV rays is also bad juju.

IF it were me, I'd do one of two things:
Upgrade to 16" wheels and 16" trailer tires that still fit under the fenders. The more wheel you have, and the more diameter you have, the less heat you have.
If you cannot upgrade to 16" setups, just replace everything with Maxxis brand E-rated ST trailer tires. They are far and away the best ST trailer tire on the market. They are the ONLY ST trailer tires I run.

My new trailer is currently in grass until I figure out a more permanent situation. What's the alternative here, putting down some 4x6s and backing on to them?

2x12s Maybe even stacked up X2.
Get it up out of the grass. Some folks even go so far as to put teflon sheets on top of the wood so that water runs off around the rubber.

I had a "new" tire blow out once on my truck, 2 months old. tread separated at 75 and wiped the bedside. I called the tire mfg. about it. They had me read the date code off the tire, 3 1/2 years since mfg. they informed me that industry standard on tire life is 3 years from date of mfg. and that the life of the tire had ended before I even bought it.

I got 8 years out of my original Goodyear Marathons. Only replaced them due to age. Trailer is never left outdoors but my indoor storage is gravel floor. Never block the trailer up. Those tires were manufactured in Australia. New Goodyears are Chinese. 4 seasons so far and no issues. Would go Maxxis next time, though. Towing home from SOTPR in LOTO with a buddy we had a tread separation on his trailer and it wiped out a fender support and a brake line. No fun.

This is the problem with a lot of "shoppers clubs" (BJs, Sam's, etc.) that sell 'new' tires. Many of them are quite old stock.
We did have a new Maxxis (200 miles on it) blow out the side wall too, but feel as though it was anomaly, and have had no problems since.

I expect a tire manufacturer to say "3 years tire life". That helps them sell more tires. For the most part, I hear and go by 7-8 years. But that does go directly to the manufacturing date on the tire, not the date on the receipt when they were bought and put on the ground.

The run flats on my Corvette were driven on for 15 years, only 1000 miles a year and still are holding air just fine. But they are now decorations with the OEM wheels on my toy box's wall.

Sydwayz is right, unless you are storing the trailer in heated floor shop the moisture still comes thru the concrete. Any kind of plastic will act as a vapor barrier and block the moisture. Wood will not stop moisture, it retains it.