First: "Is it ok to take one tank to be refilled"
If your RV has a proper tank cangeover valve (often with a Gauge that has RED and Green segments) yes.. If it has a simple "T" fitting the answer is a tad more complex but if it has the proper change over valve yes.. Just make sure the OTHER tank is selected when you do this.
2nd. IF the change over valve is working as it should when the SELECTED tank runs dry the indicatator goes red.> Flip the lever/turn the knob and take the empty to Refill
3rd
Your Hardware. Or Bar B Que place can sell you a strip, either peal and stick or magnetic. IT's pertty colored. But the center of it is an LCD crystal display.
You start by sticking it to the tank(S) then boil a pan of water on the stove, USE the propane stove to boil it.
Pour hot water over the strip.. At some point you will notice a SERIOUS color change.. That is the level of liquid remaining in the tank They work amazingly well and cost very little.