My understanding of this topic is basic but here goes:
P is for passenger, LT is for light truck. You may see either type on the same vehicle, this is to say there could be some crossover. There are a couple of things to consider - passenger tires are built to provide a softer ride and will not carry as heavy a rating if used on a "truck". Figures I have seen suggest at least a 10% derating if used on a truck. Truck tires on the other hand run at higher pressures and don't flex as much so the ride isn't quite as smooth. I have not seen any recommendations for using P type tires on a tow vehicle of the burbs size...
As far as going with 30/9.50/15 vs 235/75/15's I think the 30/9.50/15's look MUCH better matched to the burb while the 235/75/15's look more like little donuts, I have the 30/9.50/15's on my burb as well. It's amazing what about a 1/2" difference in width and diameter can do.
Since the burb is used to tow a horse trailer several times a month I suggest paying the upgrade and going the LT route so the lovely Mrs. Cussler will continue to enjoy bathing for years to come.