Ah no, don’t worry, I dont mean for you to route the external FX unit through a spare music channel.
With an external FX unit, it works (of course) by listening to a selected channel for it to get its source sound from… on the X1800, that would be selected by the rotary called “FX Channel Assign”. The FX unit then applies the echo to what it’s listening to. If you give the external FX unit nothing to listen to, by turning the FX channel assign to say… Mic…while the mic channel is off, then the FX unit has nothing to listen to so the FX will slowly fade out/reduce etc.
Another way of doing that, in case you’ve got all four music channels and the mic channels running, would be to turn the input control down/off on the external effects box itself… turn the input to zero, again the fx box has nothing to listen to, so the output effects would slowly fade away.
Most external FX boxes have an input level control, or an input switch to get the unit to look at Input A or Input B, or to look at the LINE IN or the XLR in etc etc - which is a great way of switching the input to silence. However, I’ve had a look at the RV5, and as that’s a guitar foot pedal, its not as full featured as a desktop/rack unit could be - so…switching the mixers effect selection to MIC (or any other unused /silent channel).