Not nocking yer van pal, i have one lol
Mine was the same, would happilly turn over for 10 mins before the battery getting tired.
I popped new heater plugs in, give it a good service, checked this and that and tinkered away for hours, it still left me scratching my head and my arse.
Gave the battery a good charge and hey presto!! Didnt last very long as the battery wasnt holding full charge. One new battery and job was sorted.
The thing is, if it runs then it can only be a couple of things at fault, eliminate them one by one and your left with your fault.
Have you tried wa