Are you sure nothing was left on eg lights inside ?
Batteries do give a lot less current when cold c/w warm, whereabouts are you as the U.K. has had cold weather recently.
How old is the battery ? If more than a few years it may have reached replacement time.
When was it last FULLY charged ie off the mains or after a long day time drive as just replacing starting current May require an hour’s driving rather than just ticking over and Diesel engines take a hefty current to turn over, particularly when cold….
Ours before fitting solar panels lasted about a week without charging, but once they have “gone flat” permanent damage may have been done requiring replacement.
Any typo that I make means the errorists win.
Any comments above are only MY OPINION and should be read as that.
We had the Vanbitz Relay removed on Tuesday [not coping with the Lithium Battery] and replaced with an Ablemail AMT 12 Relay/Charger that has 8 selectable charge options, for Smart or 'Dumb' Alternators, Lead Acid or Lithium Batteries, and a choice between set ups with a high or low parasitic charge. There is also a 9th Option, by way of manual intervention [e.g. when starter battery is really low] that enables you to fire a stream of 3 or 4 amp pulses from the Leisure Battery to feed the Starter Battery. I chose the Option 6 [High parasitic charge + Lithium Battery] and this fires a 3 amp pulse for about 2 seconds every10 seconds, round the clock. Then,when the Solar Panel wakes up, it replenishes the Leisure Battery. The Vanbitz Relay has to wait for the Leisure Battery to go into Float Mode before it tops up the Leisure Battery, the AMT supplies power round the clock; and they are the same price [£64.95]. Our Vanbitz Relay was fitted brand new when we bought the M/Home at the end of April 21 and has the spade connectors fitted, It just needs the supplied 3 female connectors attaching, If the OP or anyone who has a Lead Acid Leisure Battery wants a Vanbitz Relay, £30 incl P&P will remove it from my 'Justin Case' personal collection of spare kit