When the toilet is flushed, there is a sewage smell.

First, I'm not a camper tech, but I have worked on fixed and portable sewer systems. Get ready as it's a stinker.
* Clogged at toilet means your system is backed up and has dried out at toilet and or sewer system and/or it has not been drained, flushed in a long time. Either way your going to need to find an approved sewage drainage site and drain/flush your system.
with a bit of searching on you tube I found this link that may help you. Though if it does not work then you will probably need proffessional help from a RV sewer repair service.
https://www.youtube.com/watch?v=_zSpW1VwhA4
Aloha, ukeboy57

You should call plumber.
Why is there sewage smell in your bathroom

The smell might be sewage or just stagnated build up of a mix of things . try to use a plunger to make sure your shower drain is not blocked in any way , to use a plunger correctly you will need to block the drain hole and fill the shower with hot water and remove what you have covered the drain with and start plunging while the water is running down the drain , this gives maximum effect while using your plunger , some material may want to expel out the top of the drain while other material will be forced down and along the waste pipe after you have done this pour a large cup or two full of liquid bleach down the shower drain and see if this clears the problem ,because most sewer systems are independent from the waste water of sinks, showers and washing machines. Also when you flush the toilet try adding a bucket of water at the same time as you press the flusher , this will force more water than usual down your toilet helping to keep the pipes fully open , but if you notice that when you flush and add the bucket into the bowl at the same time that the water does not escape freely and seems to back up , this can be a sign that there may be blockages in the sewer pipes and you might try doing this extra water flush a few ,more time hoping you will clear your pipes adding some Landry washing powder to the bucket can have a effect on moving anything along and hot water is beneficial as well . But I think you will find that the shower drain is what is causing the odour pouring a mix of vinegar and baking soda down the drain as a regular maintenance job keeps them odour free and unblocked .

What all that means is the sewage pipe going to the street is getting clogged so either snake it with 50 foot snake or go buy so Drain-O (60 ounce bottle costs about $9) and pour 1/5 in toilet and wait 10 minutes then pout 1/5 in Bathroom sink & wait 10 minutes. Keep do that until 60 ounce bottle is empty. Let this stay in pipes for couple hours and your toilet will double flush now its clean with no corrosion, hair, cooking oil, and other debris.

The saniflow should be plumed into the soil pipe system not the general waste water system.

Sounds like you have an intermittently blocked vent pipe (the one that goes out through your roof). If there is gas pressure in the downstream sewer line and it can't get up through the vent, it could bubble up through your toilet or sinks. I would get up on the roof with a few jugs of powerful drain cleaner and dump them into your vents. You might also run a few dozen gallons of water down those vents after the drain cleaner has had time to work.
Good luck!

Obviously the drain line from your toilet to main drain out of the house ( used by upstairs ) has a blockage from your toilet line to "Y" in to main drain line. Depending on how it has been plumbed, there may be a clean-out between the 2 that can be used to remove the blockage. Your drain for shower is piped in to same line as toilet, thus when blocked at other end, overflows to shower. You can try some drain cleaner down shower drain if it will go. You might get lucky and release the blockage before plumber shows up .

You are probable getting a odor for a short time as sometimes when toilet is flushed it will break the water/gas seal for a second or two. One of the best things I have found to use with sewage and grease trap systems is called Wham. You may have to go to plumbing wholesaler to find it but it does a very good job of cleaning,dissolving,and deodorizing. It smeels to me like the old pine oil cleaners. Follow instruction on the can and use regularly for best results. Thank you.

There are only three things to flush down a toilet.. Number one,, Number two and toilet paper.
That is it ! I don't think you need to hear all the things I have pulled out of sewage ejector pits
in my day..

Personal wipes and feminine hygene products are examples of what not to flush down the toilet.

Hope this helps !

Did you see any mold? Did you see any areas that are wet? TOXIC is a term that is used to describe something that is poisonous. I think you mean noxious which means unpleasant and possibly harmful. The smell of sewage would be classified as this.