Most towables today are built with so-called Easy-Lube axles, which have a grease fitting in the axle end, and a drilled hole through to a point between the seal and the inner bearing race.
I have seen numerous problems with this system, resulting in grease-contaminated brake shoes and drums, and almost zero braking effectiveness at that wheel, which with a heavy rig, is a scary and dangerous thing. I have seen brand new units with grease-contaminated brakes (including my own fifthwheel when we took delivery). Whether this was caused at the factory or at the dealer during PDI, is open to conjecture.
With the Easy-Lube system, if the recommended procedure is followed EXACTLY, the seals are in good condition, it is not too cold, and the planets are aligned correctly, it is possible to lube the bearings without getting grease on the brakes. However, I feel the need to inspect the bearings and brakes every 1 to 3 years, depending on the distance traveled, because these are critical components. To do this inspection, the hubs must be disassembled, and at this point it is easy to grease the bearings properly while doing the inspection, and replace the seals. This way, you will KNOW that you do not have grease-contaminated brakes.
This picture shows a bearing failure resulting from lack of lubrication. This was on a 24 foot travel trailer, and the wheel literally fell off as the owner was pulling into the campground.
This failure required a complete new axle assembly, and the trailer had to be removed to a service shop on a flatbed truck.
It could have been much worse. If the wheel had separated on the highway, other traffic could have collided with the bouncing wheel and tire, with possibly disastrous consequences.
A typical Easy-Lube axle end. Note the grease fitting
This is a properly-lubed wheel bearing in place in the hub, just before the new grease seal is installed.
We use high-quality heat-resistant synthetic grease, and a quality double-lip seal.