Airstream recommends repacking the bearings every 6 months or some amount of miles that is not a lot of miles, maybe 10,000. I expect very few people do it that often. It has been 1 1/2 years and about 18,000 miles on ours and the time has come.
You can do with with a fairly decent collection of tools, but you should have some experience with tools and do a lot of research before you attempt it. There are many threads here about wheel bearings. It is not all that hard if you have some mechanical skills, but it will take a long time (about 3 hours/wheel for me) while you are learning. You really want to do it right because the consequences of not doing it right are not good at all.
With a tandem axle trailer, you do not have to use a jack. You can pull one axle up on blocks of wood and do one axle at a time. This will mean keeping the tow vehicle hitched while you are doing it. If you use a jack (floor jack is easiest), also use a secondary means of support for safety. Jack stands are best. Chock wheels on the tow vehicle.
Do not mix greases. Make sure you get the bearings absolutely clean. Then dry the bearings and do not rotate the bearings while drying—they have no grease in them. I believe you can blow compressed air across them so long as they don't rotate. Since there is no pressure on the bearings, a little rotation may not hurt, but better to stabilize them.
Some people destroy the seals getting them out and have new seals ready. It's pretty hard to get the seals out any other way. It may be possible to press them out with a dowel, but be careful not to touch the bearing because they are more delicate from the sides that you may think. It is generally recommended to replace the deals each time because you really can't tell if they are slightly bent. I have used them over after getting them out very, very carefully, but probably won't do that in the future.
All these parts are pretty easy to get from any auto parts store, but finding out the proper numbers for them can be a chore. When we had to get the parts numbers in an emergency in northern Minnesota, the nearest store—a NAPA dealer—had no idea what RV's took. At least an hour of phone calls finally got me the Timken and other parts numbers so I could order them. I believe the size of the hub (brake drum) has changed slightly in recent years. This doesn't affect brakes or wheels, but does affect what size bearings and seals. I had a hub replaced in JC 1 1/2 years ago and that one is a slightly different size than the 3 others. This will drive me crazy soon.
I can do this and have, but do not get any enjoyment out of it. If you have someone else do it, make sure they know what they are doing. Autos and trucks rarely get bearings repacked anymore, so a lot of mechanics have little practice with this. RV shops do it more often, but charge RV prices. I'd look for some old guy who has repacked bearings for 40 years and knows what he is doing. And, if the bearings are bad, replace the races—they may be scored and you can't see or feel it. It takes a special tool (actually a set with many sizes) to press a race into the hub to make sure the pressure is the same across the entire part; races distort easily and the bearings will soon wear.
Some of this I learned from my mistakes, some from other people's mistakes that ended up costing me a lot. I did the research and know how to use tools, but it takes time and experience to do it right. Most of you can do it, but be very careful. A wheel coming off on the highway because the bearings got fried is not something you want to experience. I didn't know a bearing was overheating, and stopped just before we lost a wheel because I wanted to use the bathroom (thank you bladder!). When I touched the wheel—it was at an angle—it fell on the ground. We were possibly seconds away from losing it. The damages was not so bad I couldn't fix it at a nearby campground, though we later discovered the hub needed replacing too—but we drove another 5,000 miles without incident. This happened, I believe, because a "mechanic" at a tire dealer overtightened the castellated nut thinking he was doing me a favor.
I'm not trying to scare anyone. It is one of those maintenance jobs that needs to be done very well and you must have confidence you can do it and be prepared and careful.