You are right, but current (i.e. 1997) running times would result in marginal hours at Seattle and Los Angeles if it were to be a two-day, one night trip.
So, I pulled a 1989 timetable. SEA-SLC marks off at about 23 hours (it's just a hair below). SLC-LAX is about 15 hours.
Working from LAX north, here's roughly what I get:
LAX: 0000 (0000 PST)
LAS: 0700 (0700 PST)
SLC: 1500 (1600 MST)
BOI: 2200 (2300 MST)
PDX: 3300 (0900 PST D2)
SEA: 3700 (1300 PST D2)
The above wouldn't be a bad timetable, either.
In the opposite direction:
SEA: 0000
PDX: 0400
BOI: 1500 (1600 MST)
SLC: 2200 (2300 MST)
LAS: 3000 (0600 D2)
LAX: 3700 (1300 D2)
This timetable is a bit of a mess. If you pull it seven hours earlier, however, that gives:
SEA: 1700
PDX: 2100
BOI: 0800 (0900 MST)
SLC: 1500 (1600 MST)
LAS: 2300
LAX: 0600
In both cases, I might "pad" LAS-LAX by about half an hour to get slightly better times in LA without moving Vegas too far into the graveyard.
None of this makes for a great connection with the
Zephyr, but you could plausibly pair this with one or two trains only covering the BOI-SLC-LAS segments. If you're willing to run a train on each segment solo, a midday departure from each endpoint would arrive in time to catch the
Zephyr with sufficient connection time. As to connecting
from the
Zephyr? That's a lost cause on the current timetable westbound. Eastbound...the connection works on table, but 0300 isn't really conducive to people
making that connection.