Re Tanks Ax others have sait the tanks on a TT are basically identical to a BBQ. May be bigger (30 pounds instead of 20 as an example) but otherwise identical and you can indeed swap 'em.
Re gauges
There are two types (well 3 not that I think of it) possibly a 4th I will describe
Pressure gauges. Often with just a RED/Green dial They tell you "Yes there is liquid propane in the tank" or "You just ran out" If there is a quarter cup of liquid left. they are green. The only thing that will move them towards EMPTY is a cold snap. Yup they are better thermometers than propane gauges.
Of course you know you are out of propane cuuse the furnace does not furnace and you are freezing yoru... Parts... Off.
They are useful as leak detectors however (I'll let others explain)
THe next type is a liquid crystal strip you attach to the tank, EIther peal and stick or magnetic.
You then, using propane from THAT TANK, put a pan of water on to boil (Tea kettle is best) then dump the voiling water over the strip. It will change color. and become TWO colors. the point where the color changes is where you find LIQUID in the tank (avove that is gas) If you get only one color.. REFILL TIME (unless you just dide)
Next is a scale. many variations on this but they make one that goes under the tank, the problem here is when you lock down the tank, you disable it.
In theory you could use a sonic sensor or other type of sensor to detect right through the metal (my dad used to do this with 100 pounders. he'd "Tunk" the tank and listen to the echo. He was very good at figuring out how much was left.. Me.. Not so much,)
YOu can buy a tank with a float type gauge. once you learn it these are acually rather good.
Tanks used on Motor homes are different, and often have a float gauge.