The results of a trailer bearing failure on your holiday can range in implications from a slight delay on the side of the road to the cancellation of your holiday and your trailer being transported back to civilization on the back of a tilt tray. These bearings are probably one of the cheapest replaceable moving parts on a trailer, and yet they probably do the most work, kilometre after kilometre rolling around, keeping those wheels and tyres attached to the axle.
When travelling I try and keep a regular check on the temperature of my camper bearings and hubs by regularly feeling the temperature of them. I make a habit of putting my hand on the hub area of the camper at every stop of the trip, carefully at first (just in case they are extremely hot), checking the temperature of each side and comparing the temperature of the two different sides. When doing these checks it's important to consider the type of driving you have been doing, the ambient temperature, the amount of braking you have been doing, and also to try and pull up without too much use of the trailer brakes. The best indication of how your bearings are going is to check them after about a half hour of travel on a flat road, rolling to a stop with no brake use. Remember that the temperature from the brake use will radiate through to the hub and bearings and may give you a false indication. Also those fancy hub covers which a lot of people fit over the dust cap/hub make it very hard to check the temperatures accurately, and so I avoid putting these on mine.
Once the temperature is checked, if they are just warm or slightly hotter to the touch, then that is a good sign that the bearings are doing their job correctly, however if an extremely hot hub is detected, then now is the time to do something about it, as postponing the fix may ultimately bring you unstuck with a complete failure, causing more damage to other parts, some of which may not be easily replaceable. A bearing which is running hot may just be overtight and require a slight adjustment to relieve the pressure a little, but it could also be a sign that a bearing has or is about to fail completely.
To narrow down the cause of the over temp, you can give the wheel a bit of a wobble by pushing on the top of the tyre (side to side), feeling for movement or clunking in the hub, a failing bearing will or could cause excessive play in the hub area, and thus clunking when pushing on the wheel. If you suspect a failed bearing don't travel any further than absolutely necessary, but rather check further into the problem asap.
If you suspect a loose or over tight bearing is an issue, then it's a simple job of knocking the dust cap off the hub, removing the split pin in the castellated nut and either tightening or loosening the nut, remembering that very small adjustments here are usually required, perhaps one notch or two will easily adjust a poorly adjusted bearing. Remember once back on the road after an adjustment, take the time to recheck the temps again after another 1/2 hour or so of driving, and another adjustment might be required to get things right. Once you get this adjustment right, you should have a good run.
Whenever you have the dust cap off the hub, check the amount of grease in this area, as well as what it looks like. If very little grease is visible then perhaps a bearing inspection and regrease should be considered in the near future, and if the grease has a milky (white) look about it then this is a sign that the grease has be affected by water and won't be doing it's job as good as it should be, and so again an inspection and regrease should be factored in.
The picture below shows bearing grease which is slightly water affected.