not sure why you would think it's not safe.
if it wasn't, would the manufacturer even risk putting a FW tank in the trailer?
we always filled the FW tanks in the two popups we owned over a 17 year period. especially since most of the campsites in Calif. state parks, have no hookups of any kind. never felt unsafe during that whole 17 years.
and Zoe the Wonder Dog(R.I.P.)
'07 Roo 23SS pushing an '07 Chevy Avalanche
Equalizer WDH and Prodigy BC