how do you keep sewer tank clear

never use chemicals. I drain the tank, use the built in flush, close valve and let fill up for about 8 to minutes (yes I use a timer) then dump, flush and then add about 5 gallons to the tank. We do have a water softener so that helps keep the tank "slick", sometimes I will put some laundry detergent in the tank.

No chemical, any toilet paper. Never dump before at least 1/2 full. Best to wait to dump until you arrive at the next campground. Driving will slosh the contents around, break stuff up, and help the dump process. More water in the tank is better. After a "#2 deposit", do a second "flush" after filling the bowl with water. Will help break up what's lying under the drop tube. Plenty of water is your friend. The system should be sealed and there should never be significant smells. If your toilet bowl will not hold water, it needs it's seal replaced or you will get smells. Most of the time odors are caused by a bad vent valve under a sink letting grey water stench into the RV. I don't think anything will extend the life of valves. They all quit around the 8-10 year mark. Valves are cheap and most are easy to change. Feel sorry for those with valves way back under the RV that require cutting holes in the bottom cover to get to them!

Just water. Flush it good when I dump and add some to it when it's empty. Never any chemicals they are bad for the environment plus they stink. I have no problems with any smells from my tanks.
B.O.

We also use nothing but water, use any septic safe toilet paper and only dump when the tank is near or full. 9 years full-time and no problems at all. I think many people who run into problems skimp on water thinking they will fill the tank too fast and have to dump too often.

I just use plenty of water and then I dump a couple of gallons of water down the toilet after pulling the valve at the dump station.

Then I add a couple of gallons of water to the tank after dumping to keep the seal on the dump valve working.

I don't try to keep it perfectly CLEAR. I just don't want any build up. Using enough water to dump it properly makes that happen.

Oh And, I use a toilet paper that dissolves good. It doesn't have to be the special stuff. Just grab a couple of squares of the TP of your choice, put it in a glass of water and swirl it around. If it starts to break up after a short period of time then that's good stuff to use.

Add water to almost fill the tank before dumping. Refill with fresh water, drive for a while and dump again occasionally. It’ll be “clean.”

Chemicals are NOT needed. I installed a sprinkler like device that rinses the inside of the tank when hooked up to a hose usually found at dump stations. This keeps my guages fairly honest and I have no odors. I think any RV parts place will know what you need.

We never use anything and never had a problem.

We use the thetford tank treatment. Always have, always will.
In our 25 years of TT and MH RVing have never had a tank "sludge" up or any valve problems.
I prefer to follow what the manufacturers recommend rather than 3rd party home remedies.

never use it?

liquid laundry detergent and liquid water softner

Formaldehyde? Never heard of anybody using that. I've been using happy camper lately, and it seems to work pretty good.