your trainer is just following the proceedures out of the factory manual more than likely.
He's just got a job assigned to him and you shouldn't hold that against him. Your all in this together to help keep your customers happy. Yes they train you so they can tell the customers you are trained, a selling point of course but what customer would want a reputable company sending out tech's that were not trained? I don't know of any really. :o)

As long as you follow the guidelines set forth by the factory it should be enough.
Here is what i do when i do a PM on a gas/lpg truck, or any machine for that matter.

1 - if the unit is very dirty use the compressed air and blow it off. (if extremely dirty and takes too much time it's extra cost for the time) (Also do not water batteries on electric trucks, that is customers responsibility unless it is very low, then fluid is added and there is extra cost for that too). Make sure the customer is aware of this (no suprises)
2 - inspect the unit
3 - do the normal lube maintenance required, oil and filter changes, grease it where required, and if any small adjustments are needed if any (ones that do not take up alot of time), make them.
4 - take notes of any bigger problems found and advise the customer, if safety related make sure they know this. When i run across safety related repairs i usually stress the importance of making sure this is repaired to stay in osha and ansi compliance, that usually gets it done :o)
5 - then i clean up after myself, you leave a messy work area that looks bad on you, customers like it when tech's leave the area as they found it. And the more information you can get across to the customer when advising on repairs the better they like it, no suprises ;o)