Deano, congratulations on the Airstream. You are going to like it.
The Marathons proved to be poor tires, hence Goodyear came out with the improved Endurance. If you liked your Marathons, you will like the Endurance. I purchased my used Airstream in 2009 and promptly replaced the tires with Maxxis. I was pleased with them and when it came time for new tires I went with the Carlsile Radial Trail HD that my local Discount Tire shop was selling. I also went with LRE which was a bit of an overkill for me. They will take 80PSI I believe but I run mine at 70. They are now 5 years old and I have put over 25,000 miles on them without issue. The tread still looks great but I will replace this set next spring anyway. I do have one tire that is showing some wear on the outer edge but that is an axle issue and not a bad tire.
The reason I went to these is that the Maxxi at the time was quite a bit more expensive. I think I paid all of $100 per tire back then. Don't really have a clue as to prices today.
This might be an option for you.