See short summary of tire posts on this forum, below. Other forum members, please feel free to correct or amend, as necessary.
ST = Special Trailer
LT = Light Truck
XL = Extra Load
Options for original 15-inch wheels
* OEM Goodyear Marathon (GYM), ST tires, load range D (65 psi max). This is probably what came as standard equipment on your Airstream.
* Other brands of ST tires, load range D (65 psi max).
* GYM or other brands of ST tires, load range E (80 psi max), which have slightly higher load carrying capacity. A few common brand names include Maxxis, Carlisle and Tow-Master.
* Bias ply tires with specifications similar to ST tires. (These tires appear to be used mostly on much older Airstreams, which may have come with bias ply tires; and this substitution is not common on newer models.)
* Slightly oversized XL tires, with specs similar to old load range C (50 psi max). Typical size is 235/75x15. This substitution is relatively new; and there is very little history on their use. Also, the load carrying capacity of XL tires may be insufficient for some Airstream models.
Note: There are no LT tires made in size 225/75x15; so for 15-inch wheels, selection is limited to ST, XL and other tires listed above.
Options for 16-inch wheels
Some owners have switched to 16-inch wheels and LT tires, because they are very close in physical size and rating to the OEM 15-inch wheels and ST tires; and 225/75x16 is a common LT tire size that is widely available from many manufacturers.
Note: While it appears from anecdotal evidence that LT tires may be more reliable than similarly spec'd ST tires (especially for those who exceed the ST's 65 mph maximum speed rating), many AirForum members seem to be satisfied with GYMs and other ST tire brands (in 15-inch sizes).
The most common LT tire used is the 225/75x16, Michelin LTX M/S (or M/S2), load range E (80 psi max), which comes standard on Eddie Bauer models and is optional on several other Airstreams.
It appears that numerous AirForum members have switched to 16-inch wheels and tires, and there have been no reported failures that I can recall in the past year or two. However, there is no detailed data available to support this best-guess estimate.
A few AirForum members have switched to commercial-grade LT tires with steel sidewall plies (e.g., BF Goodrich Commercial T/A's and Michelin XPS Ribs). While these Airstream owners generally feel that commercial tires provide an additional margin of protection against tread separation and blowouts; these tires are significantly more expensive, and there is currently no supporting data to justify their use. However, it appears that owners using commercial tires feel that the additional confidence and peace of mind justifies the extra expense.