I recently purchased a van second hand as well. In my case a 4 year old tandem axle Jayco Journey Outback. I did an Internet search to get some ideas what to look for.
The main items initially of course are a current Roadworthy and Gas Certificate...Even then as far as a Roadworthy is concerned, it won't tell you whether something is on it's way out and needs replacing. When i had a roadworthy done on my camper trailer the Roadworthy inspector just checked that the lights worked, nothing about the brakes or handbrake adjustment...nothing.
There can be very questionable "Roadworthy Certificates" issued...
The van i bought had a roadworthy on it, but when I lifted the wheels later to check bearing play etc, i had to nip up two wheel bearings....no big deal but should have been picked up on the Roadworthy.
The things imo to look for are basics such as damages to external or internal fittings. All lights work and when gripping the wheels to check for play that they don't move.
Internal lights and external, gas appliances operational, air cond if fitted, and check for water ingress or water stains, window locks and roof hatches close and lock...water pump operational and drains not blocked...if it has a hot water system fitted check the anode, if it hasn't been replaced for years the hot water system internally could be compromised and spring a leak meaning replacement...mostly basic stuff really.
Being second hand some wear and tear is inevitable, just it depends on what you will accept and of course the price...
No fire extinguisher, no smoke alarms? I have a check list, but you don't want to go near it.
That was a service check list Ian, smoke alarms have 10 years then replacement, fire extinguishers can only be serviced by licensed companies, not caravan yards.