As a caravan engineer I offer the following thoughts to the conversation.
I actually tend to agree that a competent owner can undertake quite a lot of the annual service tasks, providing he has the right equipment, but to buy what are expensive tools for some jobs one has to decide if the cost is effective for very little use.
It is different in my case as I use these items many times every day so the cost is not an issue.
Where it becomes more difficult for an owner to complete certain tasks is where gas and electrics are concerned.
For electrics the trip value for the MCB's and RCCD need to be measured as well as resistance of the insulation, which needs specialist equipment.
For gas there are the tightness testing, let by testing and flue gas analysis, which all need specific test equipment which is very expensive, especially for once a year use.
The whole reason for regular servicing is not just to check that things are working, but that they are working safely for the safety of the owner and his or her family.
Unfortunately the caravan world has no real organisation for the owners, just for the engineers and makers, and the makers will only accept proof of a service if it has been done by one of their dealers or nominated workshops, or by a registered member of the AWS, and I fully expect that Insurance companies will also require the same .