is it a goodyear G670 tire? Can you post a picture of the wear on the tire? THis https://dl.dropboxusercontent.com/u/201045551/Forums/PhotoUpload/RV.NET-Photo-Upload.html will let you easily post pictures here. I use onedrive, or iCloud, I take a pic and its on my laptop within moments, bing bang boom, pic is on this site.
Goodyear G670 tires are prone to "rivering" which is when 1 tread line on the tire (could be anyone, outside, inside, middle) wears quicker than the others. You can tell by looking at it, wear caused by alignment problems shows a pattern, rivering, the tread is just gone.
As stated it is an alignment problem. Easy to get fixed BUT you need to find someone who knows how to do the alignment on a Spartan chassis. Many (most) places do not and if they do not know will screw it up. Been there done that. Have a friend that had a bad experience simply because some mechanic thought he could do a alignment and probably could, just not on a Spartan chassis.