many people have problems with UVs that are incorrect ingame. there can be two different resons for that:
1st: the model has multiple clusters and/or texture projections. mutiple projecionswill mess up the model(like repsharpshooter said) and multiple clusters may. for everybody who doesnt know where to check the clusters/projections, here is a picture of how it mustnt/must look like:
2nd: the model contains some data that meshex doesnt like/understand. you can either select all models, export them as obj and reimport them or select all polygons, right click and choose "extract polygons (delete)"