If I were to buy another Mh internal hab checks I would make
Fill system with water
Visual check water tank top
Run all taps and check pump works
Check heating works plus hot water
Run heating for at least 20 mins
Check loo flushes , bowl holds water. Operation of emptying lever
Check level gauge
Quick visual for leaks including under van. (Ask to see heating dump valve )
Whiz round with damp meter under Windows ,floors and by overcab body joint
Check fridge works on gas 12v and 240v if a 3 way fridge
With a cheap plug in to ciggy socket voltmeter (about 2 quid from ebay) check cab socket goes to 14 ish volts when engine running. Likewise in hab.
Stop engine.
Check voltages when off.
Leave meter plugged in to hab.
Plug in landline. Check meter rises again to 14 ish volts
Check all lights work
Check all door and drawer catches
Open all windows .Check operation of blinds
Open and lock hab door
Swivel seats
Seat belts
Cushions, condition and present
Open gas locker. 2 bottles
Check cooker. All burners light and stay lit
Check oven and grill. Light and stay lit
Micro if fitted
Hab batteries visual. Professional installation?
toilet cassette access door opens locks cassette present?
Roof hatches operation and condition. Actually open and close them
Vehicle checks the same as per most other vehicles plus
Have a good look at
water filler
electric socket
Windows
seams seams seams
Plus anything I haven't thought of.
THEN If happy I would pay the balance of the purchase price.
Hope this is helpful