One thing I've started doing while filling my fresh water tank is to not go only by the gauge lights. You can see the fresh water tank by looking in the low point drain compartment. Just watch the water level and shut off when you see that the tank is full enough. Also, if the coach is on a small incline, lower the jacks so it's level.
As for the overflow, after the dealer put in a "P" trap in the overflow line, I've not have any problems with the water draining out while driving. BTW, FR knows this is a problem and will pay for the fix.
I hope this helps.
Smooth Roads, and, Drive 'em Safe
Curt & Isabel, & 9er (our Goldendoodle)
2014 Forest River FR3 30DS (Beasty)
2014 Honda CRV EX-L (toad)