I don't carry grease gun any more. My road "ER kit" has a set of packed bearings in an old plastic butter container and inside a plastic bag. I use cheap Walmart "syne tech" because you can get at every Walmart nation wide if I need any. Used to use Ford racing high temp grease with very high "dropping point" grease at $17.00 bucks a tube. Not anymore. If you see high temps you have other problems no matter what type of grease you have. I have not seen temps over 160 on our hottest days.
IMO Something else is going on if your repacking every spring and are still having problems.
I would start at the axle area. Hopefully you have an axle sticker on your axle tube to look up your axle spec's. Making sure you have enough weight carrying capacity and 10% spare capacity for the weight your placing on it. If tire wear is normal that means your trailer is pulling somewhat straight.
http://www.amazon.com/Milton-S448-Tread-Depth-Gauge/dp/B0002STSQM If bad wear on tires, and you have been using the right PSI in tires, you could have a bent axle or a spindle that might on been put on wrong from day 1. Then spend some time at a place that has cross reference on bearings (not your normal auto part store), look up what you have and get biggest weight spec you can get for your size. I never had a problem with import bearings, even though I try to buy US made. :) to match my offshore tires and rims, lug nuts.