You do not want explosive gas vapors inside a RV or inside the towed vehicle. That leaves mounting the can on the RV or on the tow vehicle. Easiest would be a military style jerry can with a mount welded or bolted to a bumper. These are easy to find as they are a common CJ accessory and they gas can is easily padlocked inside the bracket.
FWIW, not sure if it's safe or not but after we ran out of gas a few years ago, I travel with a 5 gallon jerry can with 2 or 3 gallons of gas in it in one of our storage bins. I've actually never really thought if it was safe or not but I've never smelled any fumes from it. Once or twice a year, I empty it into our tank and refill it with fresh fuel just so the gas doesn't get too old.