A while back I bought a Pro-Fill system from Flow-Right
https://www.flow-rite.com/battery-watering/marine-rv
https://www.flow-rite.com/battery-watering/marine-rv/pro-fill-rv/installation-operation
I wrote a post here (somewhere) praising it a while back but darned if I can find it now.
My 2 banks of 6v Trojan T-105's weren't impossible to maintain but it was a chore. Each bank (of two) is installed just ahead of the fuel tanks so the battery most forward, closest to the bulkhead (esp to port) was difficult to reach. Thus I would constantly put off topping the batts.
The Pro-Fill makes it so easy. I cannot recommend the Pro-Fill system enough for those with wet cells in hard to get to places. No cheap but cheaper than buying new batteries because the chore of maintaining them keeps getting put off.