Factory tire pressure for P215/75R15 SL was 28F/32R.
At 28 psi, that size tire can support 1561 lbs. At 32 psi, they can support 1664 lbs.
With an LT tire, even at the same size the load is drastically changed. LT Metrics aren't even rated below 35 psi. Also, with passenger load tires, you add in 10% overage in load capability because they're weaker tires. So if you needed the stock tire to support 1500 lbs, you would inflate it to a pressure that supports 1650 lbs. When running LTs, you remove that overage factor.
So, you have to take out that overage for your LTs, meaning you need an LT tire to support 1419 lbs up front and 1513 lbs in the rear instead of 1561 & 1664 lbs.
Next, you have to look at what weight an LT C tire in the size you have can support at what psi. LT215/75R15 C is rated to support 1397 lbs at 37 psi and 1423 lbs at 38 psi. So, technically, you should be running 38 psi in your size up front to replace the stock P215/75R15 SLs (remember, you're trying to match the 1419 lbs required which came from adjusting the stock pressure and reducing it by 10%).
Same deal on the rears, at 41 psi, LT215/75R15 is rated to support 1501 lbs, and at 42 psi it's rated to support 1527 lbs. You want 1513 lbs so you need to go with 42 psi..
So technically, you should be at 38F/42R.
You might find that a bit harsh in ride quality, due to the stiffer LT tire, but those are the pressures where they will have the same load capacity as the stock tires at the stock pressures, and should theoretically wear the same. Of course, you may be able to get away with less pressure and no ill effects, but no telling for sure on that.
The owner's manual also says to bump the pressure up 3 psi if you'll be driving at more than 74 mph continuously for a while, like a road trip...in that case, you'd want 41F/45R psi, based on the stock pressures being upped from 28F/32R to 31F/35R.
You could play around with it some and see what you like but my guess is currently you're significantly underinflated. Sometimes though, it really doesn't matter so it may just be fine.
This tire pressure calculator website is a really handy way to calculate what you need based on the OEM recommendation. The trick is using it right. If your stock tires had a P in front, you need to choose P metric. If they were just the numbers, you choose euro metric. If they were LT metric, you choose LT metric, and if they were high flotation (31x, 33x, etc), you choose "LT".