It is safe and in most places, legal. Tunnels like the Chesapeake Bay Bridge / Tunnel are an exception, but are usually posted before you enter them. I've towed with my refrigerator running on propane for years.
Mike and Yvonne
and Sophie, the little white dog
2017 Columbus 320RSC
2015 Chevy Silverado 2500HD Duramax 4X4 “It's not how old you are, it's how you are old.” ― Jules Renard "It's not the years...it's the mileage." - Indiana Jones