When it hasn’t started
When it hasn’t started before, have you got a set of jump leads from another vehicle and put them on the vehicle battery to see if it starts? If not, try it. Don’t mess about with the leisure battery, leave it. Just see if it will start ok with a jump from another car. If it starts ok, take the leads off and then switch off, leave for an hour then try and start it again if it doesn’t, try jump starting again from the other car, if it starts, I reckon your vehicle battery is F****d.