The number 1 cause of early corrosion in boat trailers is ....lousy glavanising... and Dunbier suffer from this problemt at least as much as any other manufacturer.
When you buy galvanising, directly or indirectly you pay by the kilogram for zink deposited.....the trailer business being a competitive one, all trailer manufacturers will be trying to control costs of galvanising.
Have a good close look at a few boat trailers, then look at some structrual steel in application where long term performance must be guaranteed, the difference in the quality and thinkness of the zinc will be obvious.
If you look at a few new boat trailers you will see faults in the fresh new galvanising that would be rejected in any decent engineering project...faults like dross ( black brown or white deposits)on the surface, pitting in the film, roughness, failure to wett out, and failures to penetrate holows and into crevices and corners.....you will be hard pressed to find a boat trailer without some of these and other faults.
the thinner the coating the harder it is to get a smooth even job.
Lanolin has a very good track record, I can not think of any possible reason why lanolin would promote corrosion.
Probaly one of the best things you can do is once your trailer has aged and the galvanising has " gone off", is to paint it with a good quality paint designed for overcoating galvanising...galvanising is a sacrificial coating and sooner or later it will consumed.
As far as the bearings.....there is all sorts of fancy things you can buy, but there is no substitute for regular repacking and water resistant grease.
There have been big changes in grease in the last few years and lithium complex grease ( blue grease, castrol LMX or boating grease) is far more water resistant than greases of the past.
Old style lithium ( castrol LM) and bentonite ( castrol HTB) have almost no water resistance, just a little water mixed with these and they will start to break up and the lubrication capacity fails. Aluminium complex grease, the last generation of waterproof grease was heaps better, but lithium complex leaves it for dead.
Almost every petrolium company makes a lithium complex grease and almost all are blue.