Is that just a dust cap on the end? If so, how about popping that off, and slopping grease in with a butter knife or something? (Can't tell where the grease nipple is from the pic, and not familiar with Indispension trailers.)
Alternatively, you could pull the hubs, clean and repack the roller assemblies, repack the hub, and put it all back together. Lot messier, though.