It all depends on your individual situation. If you put a lot of highway miles on your trailer, you need to repack a little more often than normal. If you immerse your hubs in water, you'd be wise to repack. If you are only going 5000 miles a year, it is not necessary to repack every year.
EZ Lube hubs shouldn't even be on a travel trailer or 5ver. As mentioned above, they are for boat trailers that get immersed in water. I had some questions about the EZ Lube hubs on a toy hauler I had, and called them. Their representative said he gets a lot of calls from RVers that have blown out the back seal. He says that the bearings on a non-boat trailer should not be lubed via EZ Lube. Take the hubs off and do it the right way. Its a very easy job, the hardest part is jacking it up correctly and safely.