NO IT IS NOT SAFE TO ASSUME THAT THE HOT WATER TANK IS FULL. I did this when I got mine last spring, even had water coming out when the hot water handles were turned on. I turned the propane on, it lit, and my element was toast. Replaced it for twenty bucks or so.
To check your valves, you will have to either cut a hole in your plywood under your queen bed to access OR as others above have done, take the screws out and lift both your mattress and plywood up. On the right side you will see your hot water tank and the 3 'mystery valves.' The handles on the
hot and cold lines should have their handles
parallel to the
water
lines. There is another line that runs between these two that handle should be
perpendicular to the line. Make sure your handles are in this position before firing up the propane or hooking into 110.