I agree, when it comes to the manual, it's not very helpful for maintenance!
Our dealer included a list of maintenance items with the documentation for the trailer. In addition they covered a lot during the PDI. They also have occasional free seminars on things like winterizing. Using resources like this, and things others have mentioned you can build your own list.
Keep in mind too that some things may be specific to features your trailer has. For example we have a small slide out which has it's specifics, and being a hybrid, we have the bed seals to maintain.
Quite honestly I don't find there is much more to do annually than what you already mentioned, and winterizing/de-winterizing. I would also add checking all seals (windows, doors, roof, cord receptacle, running lights, etc.) for dryout and/or cracks. Checking the spare should be done before every trip.
2013 Jayco Jayfeather X20 E, 2008 GMC Sierra 1500 Crew Cab Z71 SLE