Ive always raised eye brows when I comment that mine is happier sitting at 80 mph ( sat nav verified ) on the motorways , as this is considered to be revving the engine to high, I dont have a rev counter but always estimated the revs to be between 4-4.5 k rpm. well within the safe working speed of the engine.
I have 25 years plus experience in the Motor trade and have driven most things, its quite easy to get a 'feel' for a vehicle and and what revs the engine is happy at... get 2 identical vans and they may well feel different, a lot of it comes down to how they where run in right back at the beginning. There are 2 different schools of thought about this, some say gently gently , where others say build it up to full revs quite quickly.
Everyone knows their van better than anyone else.... so listen to how yours sounds and feels, it will let you know if its not happy. 5400 rpm is the safe max speed , it was designed to be able to do this. My motor bike would rev to 15000 rpm but I wouldnt keep it there all day long

just as I wouldnt hold 5400 rpm in the van for long, if ever ... there is bugger all power up that high so no point.
2.5 Quad Cam Scooby engine . Westfalia California 1989