What's that smell??

Close the valves. You become the vent for the sewer.

It could be the valves left open but usually the roof vents take care of the sewer smell.The P traps under the sinks are filled with water which makes a seal that stops the intrusion of smells into the rig. However under each sink you will find a little vent
Looks like this


They may be bad allowing smells to back up into the rig. They are cheap and easy to replace. Home depot, Lowes etc under $6.00 each.
To troubleshoot I would take a hose onto the roof and put water down each plumbing vent to make sure they are open. If water goes down okay then I would change out the under sink vent(s).
As for the 2/3 full problem search for the "geo method" and fill your tank up. Let it sit then dump. You can buy a hose wand to spray the inside of the tank and if you are lucky (very lucky) that may clean off the sensor enough to get it working

OP, you're saying you have 6 waste tanks and the fresh tank as well for a total of 7? That sounds odd.

The 2/3 reading is pretty common even after dumping. If you know the capacity of the black tank, add "X" quantity of water after dumping (we add 10 gallons), then just kinda keep in mind how much water is going into tank or how often the toilet get's flushed. I agree w/darsben on the "geo" method. May not totally clean the sensors, but will make inside of tank slicker than snot on a door knob.

If your Sandpiper Command center is like ours you have a switch saying 1 and 2 next to your tank indicators. The switch does nothing. You have one grey tank for washer, shower and bath sink and one galley for kitchen sink and outside kitchen sink if you have one along with one black for your toilet.

Our command center quit working this spring. When toilet starts to burb when you flush it I know it's almost full. Leave your grey and galley tank closed and empty when full. I pour a cup of bleach in the grey and galley tanks a couple times of year to kill the bacteria so they don't smell.

Congrats on your Sandpiper. We are on our second Sandpiper/Sierra and love them. Have had very little issues. Good luck.

Didn't see you mentioning leaving the black tank valve open but, since you are new, I'll mention...DO NOT leave the black tank valve open!! EVER! You will, if you haven't already, experience the "Pyramid of Poop". You may want to look down the toilet with the valve open to be sure the previous owner hasn't left you a 'present'! RVs are NOT like your house with a nice 1/4" in 12" sewer line slope that insures solid waste is carried away with the liquid. Sewer waste is just dropping into a relatively flat tank. RVs expect a closed black valve that only gets opened when the tank is close to being filled so when draining, the rush of liquids carries the solids out with it. With the black valve left open, the liquids immediately drain away leaving a mass of solid waste directly beneath the toilet opening until it reaches the toilet itself. You will never have a nastier job to deal with. There are hose extensions with right angle bends at the end to help break up the waste but still no walk in the park. The bad reading on the gauge is caused by waste stuck to the sensor. Best way to remedy that is dump some dishwashing soap down the toilet and take a drive with a nearly full black tank to slosh it around with the hope of dislodging the waste on the sensor, then dumping. AGAIN, DO NOT leave your black valve open! Keeping the grey valve closed is also a good idea so that once you dump the black tank, you have some less nasty fluid to flush the sewer hose out with.

See through holding tank experiments:

Video playlist

I find our galley tank to be smellier than the black tank. I always leave the valves closed - you get a better clean when a large volume of waste rushes out.

Ignore the 2/3rds indicator light - those things are just for decoration.

As stated, you have "HOLDING" tanks, they are designed to hold, "stuff", until full. Then you dump them. Always use plenty of water, black tanks start with about 7 gallons, then make sure you use plenty of water when you flush. As far as the faulty indicator, it can be frustrating, after using the RV for a while, you get a pretty good idea of when you need to dump the tanks. You may try the "George" method, your starting with the 7 gallons in the black tank, a 5 gallon bucket fill with hot water, add 1 cup calgon water softener, and 5 drops of Dawn dishwashing soap. Flush that down the toilet, then drive to your next destination. The sloshing while driving should help clean the sensors in the black tank. First thing when you arrive, dump the back tank, use the sprayer, dump again. Your sensor should now read empty. If it doesn't you may have a bad sensor. Good luck!

One problem I noticed in the video noteven had link to was that the "test" tank was tall. RV tanks are flat and just e few inches tall. May not make a difference, but in my illogical thinking, there is more water pressure w/ a taller tank than flatter one. We always add 10 gallons of water when done dumping plus capful each of Calgon and elcheapo fabric softener. Everyone has their own method of how they do things, I use the black tank flush by closing valve and let run for 2-4 minutes, then open valve. Repeat a couple times until discharge water is pretty much free/clear of anything (clear elbow adapter). Haven't ever used the device that attaches to discharge pipe that back flushes from end back into tank while still utilizing the 3" hose.

In addition to the 10 gallons we add, our rule of thumb is 1 flush for #1 and 2 flushes for #2 (but add water to toilet both times). When we boil water for corn on the cob or salt potatoes, we also dump that water into black tank after it's cooled. Like it has been said many a times, water is your best friend in the black tank.

Here is a different take on your situation. Over the years I have helped several folks that thought they detected sewer odors. Turned out the house batteries were dry and trying to charge. Can be a dangerous situation.