where/how do you find a water source to fill the tank once closer to your destination?
First off you appear to have a well built Arctic Fox. Full, 1/2 full, empty isn't going to matter up to the limit of your truck. Now if you had a 22' International Airstream that is known for busting the weak frame due to the water tanks all the way in the back... You don't.
Plan ahead. Where are you going to stop? Look these places up and see if they have water available, now*. Some campgrounds you fill up on the way in. Others you fill up 50 miles before you get there. Aka going to The North Rim Grand Canyon ... due to a pipe break they recommended filling up at Jacob Lake 50 miles away. At least that's on the way. Oh, you blew a damn ST trailer tire and are running behind? Guess what: some places have hours of operation and you arrive after hours all like SOL. Commonly water would be at fuel stops and RV parks. Anywhere close to your destination will do. *Now: as some places do not have potable water that did in the past.
Some use the water tanks to keep the weight down. Then fill when they get there or close by.
My 2003 was short 500 LBS for the ability of my AF 27-5L. It's 80 Gal of water is 666.4 LBS of the total 13,000 GVWR. So if I ever pushed the limit of stuff in the trailer running low water was a way to keep it light enough. The less weight the better for tires. Some grades here in AZ will make you feel that 1/4 ton of extra weight.
This said there are some good reasons to fill the fresh water tank. Also have some bottled water on hand.
1) Everyone assumes they are gonna make it to the destination. Roadside assistance is for more than blown ST trailer tires. You may be stuck somewhere like a repair shop for hours or days. Water may be 5' further than your hose reaches if it's available at all.
2) No specific day to be there. May stop somewhere overnight and take Three days instead of one to reach a place.
3) North Rim Grand Canyon is the best example:
"Water Availability: Pipeline breaks are common, and Grand Canyon National Park recommends that hikers and runners be prepared to filter or disinfect creek water along corridor trails. Water at 1.5 Mile Resthouse, 3 Mile Resthouse, and Plateau Point has been shut off for the winter. Water is still available at the trailhead and Indian Garden." As I noted it's 100 mile round trip if you get there and the pipe broke, again.
4) My POV is the dessert. Better to have water than "HOPE" they have it anywhere on the way. Well water, if the well hasn't dried up, isn't the best quality water in some places.
5) The RV is a pit stop on the way so I always have some water in the tank for flushing.
6) Skip the long process of filling the tank when arriving after a long day on the road ... and the lines. Sometimes there is a line for the slow fill process. Pumps are slow and so are people on their hoses.
Aside of the RV water system disinfection procedure... I add some chlorine if I fill from well water. If I run out of bottled water I boil water from the freshwater tank for drinking. Giardia is no fun.
Black and grey water is best dumped when you leave the campground, some fuel stations let you dump for free, our local sewer treatment plant has a free RV dump if you have a local bill from them. Some National Parks have a dump fee. The dump by the diesel pump in Flagstaff is my favorite as I can top off fuel while dumping the tanks. National Parks can have trouble and be unable to let you dump sometimes. I have run into this. It sucks when you go from hot weather to a hailstorm and the warm/hot near full tanks "heave" and smell bad...
Smell... Look up Air Admittance Valve and replace your cheap RV unit with a better Studor Redi-Vent Air Admittance Valve Model #20362. (At The Home Depot) Generally need 2 of them: 1 for under kitchen sink and 1 under bathroom sink. Maybe 1 more for optional washer dryer area?