You might try the tiffin rv website "http://www.tiffinrvnetwork.com"
Our Allegro has an access panel from outside, but I haven't removed it yet. It appears to be a vent for the fridge but maybe there is access to H2O line for icemaker there.
The owners manual may have some info too... but I don't see it.
https://tiffinmotorhomes.com/owners/owners-manual