Trouble starting when the engine is hot.


Hi guys- I need to ask for help! My Wolf Ambulance will not start once the engine is hot. 
It typically happens at a filling station for example. After filling up the engine will not turn over, just completely still, apart from one click. Waiting for half an hour will sort it out most of the time, as in-it will start immediately and normally. 
This has been happening intermittently from the day I bought it in 2016. 
Here is what I tried:
New batteries (My wolf has two 12V in series) I think it made it slightly better, but it still kept happening.
Freshed up all the earth connections and renewed earth cables. ( Made no recognisable difference)
Installed a new starter motor, that sorted it out completely! (Cost me an arm and leg though)
I had it in winter storage in a barn, where the solar panel did not charge it. 
Batteries were dead flat when I wanted to take it out of storage.
Used the NATO-socket to jump start it with a booster. Since then the problem has returned.
Its driving me slightly mad, I can’t understand why a warm engine is more difficult to start than a cold one.
Any advice gladly appreciated!

Join to automatically receive all group messages.