Note what you said here though - "forced to retreat". Not rest. Not replenish resources on their own. Again, if you're forced to retreat, then random encounters become moot because you've left the dungeon and re-entered civilization. The non-casters don't need to rest because resting does nothing for them.
Yes, "retreat," not "withdraw." As in, we retreat to find safer spot to rest than your point of deepest penetration...which is not necessarily out of the dungeon or wilderness. The alternative is to rest where you are clearly NOT safe, low on HP & ammo and get killed. In fact, full retreats to civilization are rare, and camping behind enemy lines is our norm once adventuring.
You'll have to refresh my memory I'm afraid on the fatigue rules
1Ed DMG p69: no hard & fast rules, just guidelines (at the tail end of rules for pursuit) for the DM to use his judgement, including such penalties as slowing movement. "Fatigue merely slows movement and reduces combat effectiveness. Exhaustion will generally require a day of complete rest to restore the exhausted creatures."
As you might expect, 3.5Ed codified this a mite.
3.5Ed DMG p300-301 as well as PHB p308:
Fatigued: tired to the point of impairment. A fatigued character can neither run nor charge and takes a -2 penalty to Strength and Dexterity. Doing anything that would normally cause fatigue causes the fatigued character to become exhausted. After 8 hours of complete rest, fatigued characters are no longer fatigued.
Exhausted: tired to the point of significant impairment. An exhausted character moves at half speed and takes a -6 penalty to Strength and Dexterity. After 1 hour of complete rest, an exhausted character becomes fatigued. A fatigued character becomes exhausted by doing something that would normally cause fatigue."
There are several things that can cause fatigue, most famously sleeping in armor (see Endurance feat), failing a Con check on a forced march and traveling in certain terrains (like high mountain passes).