jean3....I'm guessing the same as others and that you left the grey tank (sink and shower water) valve and the black tank (toilet tank) valve open for four months. Since the shower and sink water has no solids, it drains without issues. The black tank obviously has solids in it and it requires flush water and urine to keep it in a liquid state so that it will drain. The black tanks are designed to be kept closed until they're full and then dumped. In your situation, you would probably need to leave the valve closed and dump once a week if you're living in it full time.
You now have four months of solid debris that will take some work to remove. Many trailers dump from the toilet bowl straight down into the sewer tank. You need to check and see if yours drains directly into the tank. You'll probably need to look with a flashlight. If it does, find a local RV shop or you can even order it online and buy a toilet cleaning wand. It's just a 3' pvc pipe that attaches to a garden hose and has a sprayer on one end. The tool is used to stick down though the toilet bowl opening and then spray around inside your tank to clean a tank with normal debris. In your case, it can be stuck down in the tank and used to flood and slowly break up the debris.
If your tank has an angle to the drain, I would suggest shoving a garden hose down the toilet and hopefully into the tank. Since it's a hose you probably won't want to use again, cut the male end off so it doesn't get hung up going in or during removal. You can use it like a snake and then start flooding the tank with water. This will probably take two people to do or have a shut off on the hose so you can shut it off if the tank starts to overfill. I would leave the black tank valve open so as you do your work, the water and debris will drain. This may take a few days of flooding and soaking to get everything out.
Good Luck
PS...Buy gloves!