There are those that travel with FW tanks empty, and those that travel with them full. I've heard many of the arguments to travel empty, but I have always traveled with full FW tanks whenever we dry camp or boondock. Never had a problem yet.
If you put inexpensive irrigation on/off valves on your FW overflow tubes, they will prevent water siphoning out when you travel. Just remember to open the valves while filling the tanks, and when using them at your campsite.
The only time I close these valves is when traveling. Otherwise, they remain open.
2019 Jay Flight 264BHW, Rocky Mountain Ed.
2020 Ford F150 XLT SuperCrew, 3.5L EcoBoost w/Turbo