Hmm, that's a good question. Luckily you know the number. They had a couple hundred different key numbers, and a few different blank styles.
Have you tried contacting Tri-mark directly?
The best option I can think of is take the lockset out of the door (six screws) and take it to a locksmith or lock and key place. They should be able to make one for you, Ilco makes blanks that fit these locksets. Might need to leave it with them overnight at worst case.
You could also try going to a local RV dealer and ask if they have one. A parts guy at my local dealer had a huge collection of these, from keys that people left there over the years.
2007 T-286SR Cherry/Granola, #6236, original owner, current mileage: 9473.8 (as of 6/18/21)
1997 T-2653 Blue Denim, #5471
1979 12 1/2' MC, Beige & Avocado, #4639
Past Sunlines: '97 T-2653 #5089, '94 T-2251, '86 T-1550, '94 T-2363, '98 T-270SR