What model year 1550? They may have changed the size along the way.
The older brochures use to list the tire size. On the top of the forum are tabs. Look for the one called FILES. That is a hot link. When logged in, go in there and look for brochures. Then look for your model year and download a copy.
The older brochures list the old tire sizes. They can be converted to the new tire sizes. Many online lists which show the conversion. If you need help, let us know.
Hope this helps
PS, Odds are high you are going to need a good sized drop shank or drawbar to get down low enough from the Expedition to the trailer ball coupler to tow the camper level. If the trailer is still set at its original height, that could be 16.5" ground to ball coupler. I just helped another member on this on her 87 T1550. She has a smaller truck but the same height problem. Hitch height on page 3. See here http://www.sunlineclub.com/forums/f7...voy-18355.html
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
Google Custom Search For Sunline Owners Club