See here. Chances for FoW are 50/50 by default for any skirmish (battle map type 2). (That piece of code corresponds to FE8U:08030E54).
That’s it for the ASM. For anything that breaks this rule (you mentioned the Ch18 map), it will be handled by events.
Specifically in the skirmish beginning events located at FE8U:089FFC30 (which are common for all skirmishes). It’s a bit of a mess at first glance so here’s how this scene works:
It sets up a list of chapter id/event scene pointer on the event queue.
It calls another event (FE8U:89FFC0C) which looks through the queued list.
It calls the event that corresponds to the current chapter/map id.
If you look at the enqueued list, the one scene corresponding to Ch18 (chapter id 0x12 or 0x1F, both point to the same scene anyway) is at FE8U:08A0013C, which looks like this:
Events
SVAL s2 0
CALL $9EE828 // Setup unit load count based on difficulty
SVAL sD 0
SVAL s1 0x32; SENQUEUE
SVAL s1 0x19; SENQUEUE
SVAL s1 0xF; SENQUEUE
SVAL s1 0x5; SENQUEUE
SVAL s1 0x5; SENQUEUE
CALL $9EE84C // Setup random position probabilities
FOGVISION 0 // Force fog vision to 0 (no fog) (!!!)
END
There is your one exception.
(So no, there’s no fancy table mapping chapters to FoW chances, only the default 50/50 and then it’s up to events).