Technicial answer: Does not matter, propane is a sealed system so it can NOT draw moisture for there is no moisture laden air in the tank.. Gasoline (And Diesel) fuel tanks are NOT sealed however and should be full to prevent condensation, But Propane, no such problem.
Practical answer: FULL, Reason: Well.. The following is a true story....
Came home from Chruch one Sunday morning to find Frosty the Air Blown inflatable snowman sleeping on the job.. If you guessed that Detroit Edison had fallen down on the job and the neighborhood was without power.. You guessed good.
It was New Years Eve. So you know it was a bit chilly out,, and in by the time I got home from Church (Long story but Church was 4 Hours for me starting with a pre-church Breakfast with the Music Director and lead Tenor, nd then two services (I was part of the Music Ministery, ie: Choir) then home.
Now in this case, Since I modified the Motor home slightly (added an outlet for this job and a couple of breakers (30 amp) and the house (had a proper generator transfer switch professionally installed) the motor home powered the furnace (Freezer, Fridge, TV's Computer, Microwave and selected Outelets and lights, But not one KEY light) so I did not need to move to the RV to keep warm.
But in your case. Full Propane tanks means you can keep you warm and food cold by moving to the RV when, not if, that happens to you as it did again to me last weekend. I never fully lost power (Since I'm in a Class A) many did. (Thousands).
Oh the Frosty story:
Click: Lights
Click: Furnace
Click, Click, Click and so on: all the other stuff I powered.
Click No light in hall (The key light)
All this hardware: expensive
Warm wife: Priceless.
Why the key light did not come on... It was not wired through the transfer switch, only to the main breaker panel and mains power.. When it came back on.
Click click click back to mains power and go out and shut off the generator.
Today, in the Class A. I have a small fan that works... The same way.