Solar panel and leisure battery charging

A 2 year old battery,  is flat totally.
I have a 100wat solar panel fitted.
At present drivers seat is out and battery is on charge.
It is charging slowly on the charger,  but did not hold the charge when the van was plugged in.
Any ideas as to why this battery has lost it's charge.
I would like to go to some THS  music festivals.

Have you checked the fuse between the leisure battery and the rest of the system?

Typically they can recover some of their capacity, but if it has spent any length of time (i.e. days rather than hours) heavily depleted or flat then it will probably need replacing.

Thank you yes checked the fuses.
I am going to get a new battery, It shouldn't of gone flat as there was a solar panel fitted before this battery was fitted.
The big problem is having to take the drivers seat out to get to the leisure battery.

Once you have a new, recently charged battery fitted, test each of the 3 charging mechanisms.

E.g. Cover the solar panel, put a voltmeter on the leisure battery terminals (if one not already fitted within the control panel) - voltage should be 12.6-12.8V. Uncover the panel (in sunlight, of course) and check that the battery voltage rises to at least 13.8V. Do the same for the other 2 charging mechanisms.

That way you know that the battery is actually being charged!