just brainstorming...and maybe a crazy idea
but maybe you could install a hatch, something like this
https://www.westmarine.com/buy/barto...54?recordNum=1
strategically located.
either
a) it might help with an occasional cleanout
and / or
b) which is what I would hope for if I were you, maybe it would give you access enough to jury rig a fix
I'll qualify that I think it would have to be up very high on the tank, or even in the top