20 gallons? - Damn!
Safe - a relative term
Strapped in near the back window?
(which broke open during the collision)
or........
Strapped in near the rear bumper?
(which crushed / split the cans open during the collision?)
Most probable - not a problem
Worse case - you don't want to know. Won't matter anyways........
Statistics are a bitch - especially when it happens to you!
FYI - Long term (6 months +) - plastic cans "breathe" and the gas will go bad sooner than metal / sealed NATO type cans.
Disclaimer - I have 2 ea. 5 gal. metal NATO cans secured in HD brackets midway between the wheel wells and the rear tailgate in the bed.
(SuperCab - smaller tank)
With a 2.5 lb. extinguisher secured inside and "reachable", and a 5 lb. extinguisher secured in the bed near the cab - though not necessarily for the gas can aspect.............medic bag and entry tools as well - always.
FYI -
Fire sucks - and hurts more than just about anything else I have experienced.
Be safe!