Attempting to answer in order, the engine fails to turn over. It might move a quarter turn if you are lucky. Charge it over night and it starts a few times then runs down.

It is nor holding power to start the engine

My bus number (if any): RCL2259

If 'starts a few times' really means turns over a few times rather than the engine fires and runs, then the symptoms you describe more or less match: 1) one, or two, faulty batteries. 2)a faulty starter motor. 3) an excessive resistance in any of the connections or in the cables between the batteries at the rear of the bus and the starter motor at the front of the bus. 4) a mechanical issue causing the engine to be difficult to turn over.

If you hadn't mentioned charging the batteries overnight the possibilities might be extended to insufficient output from the alternator or a parasitic loss over hours. Dirty batteries and especially damp, dirty batteries can discharge themselves over a few days. If the vehicle has an analogue tachograph which has been left parked with the mode switches pointing to anything other than rest then the batteries will discharge remarkably quickly.

It is also worth noting that there is a quite marked increase in cylinder surface area between an AV690 and AV760 and even more so to an AV590.

Start as David said and described by properly charging and load testing the batteries. An infra-red thermometer gun will help to locate a poor connection without the risk of burning one's hand.

Many thanks for all your positive suggestions. Found the most powerful 655 batteries, 142Ah, installed them and they work. Engine starts no problem. Am posting another thread to isolate a further question.

My bus number (if any): RCL2259