I have a 110 amp h leisure battery and have never blown a fuse in 5 years. My fuses are 15amp.
I have a volt and amp meter on my system and even when the battery is half flat, the initial charging surge rate is around 13 amps for a few moments and quickly drops to around 8 - 9 amps within about 5 seconds.
So it appears to me as though the limit of charge my leisure battery will accept is around 8 amps. It is a known fact that leisure batteries require a trickle charge.
However, if you are running other power hungry stuff with the engine running then I could see an issue. In my case I have a diesel air heater which is also driven by the leisure battery. If I start the diesel heater with the engine running, it will draw about 10 amps heating up the glow plug. If the battery in also drawing 8 amps to recharge at that time, then I will be drawing more than the fuse is rated for so will blow as I am drawing 18 amps from the alternator.
However as you are covered with a 20 amp fuse, I can only suggest that something has shorted out and caused the fuse to blow. I can think of no other reason for there to be an issue.
If you have an ordinary car battery to drive your pump/pumps, then I understand that a starter battery will accept a faster charge than a leisure battery will. If that was flat, then it could have taken a charge higher than 20 amps.
If you have replaced your fuse/fuses, then is the split charge relay now working? I had a blown track on a split charge relay on my son’s van which blew a fuse. We had to replace the relay as it was knackered. Has the auxiliary battery got a clean earth to the van’s chassis/body? (By a clean earth, I mean the paint scrapped off around the eye terminal to ensure a good contact.)