DAF 150 Horsebox wont start, batteries?

I will try to condense this as its turned into a saga but took my P reg horsebox to a reputable garage for an oil leak, said I wasnt in a rush for it and 8 weeks later I still havent got it back because when they finally looked at it after 5 wks! it wouldnt start. They were saying it needed an auto electrician etc but they hadnt used the isolator so I suggested the batteries had gone flat, which at first they dismissed. I was that shocked it hadnt started I called down to see for myself and it did nothing, no clicking, no dash lights, just the tacho light flashing and the horse camera came on for about 10 secs. They finally agreed to charge the batteries and the next call I got was to say they had bumped started it, that it still didnt start from the batteries and that they were 100% it was the starter motor. TBF it did have an intermittant fault on the starter as it sometimes made a bleeeeugh noise but then when tried again would fire up straight away, so not being mechanically minded I couldnt really argue that. So another week has passed and Ive heard nothing and am fearing the worse but wanted to ask before I ring yet again if my batteries have been left to fully discharge could they be totally dead despite being charged, ie basically not be holding a charge? Its just so odd this, the wagon didnt go with a starting issue, it had an oil leak from the sump. I really dont know what to do next as they dont seem to be getting anywhere.