I have done some work on my Thetford toilet, most recently it was a leak from the area you mention. It appeared to be the elbow but it turned out to be the rotary valve. All you can do without removing the toilet is to tighten connections. If that doesn't do the trick, there are two bolts holding the toilet down. Remove them and disconnect the water line. You can then easily access the valve if that is your problem. I positively identified my valve as the leak before removing the toilet using a flashlight and telescoping mirror. I was going to replace the valve but disassembled it instead and replaced an O ring. My valve would have cost about $25 new. If you are handy this is not a hard job. If you want a new toilet they are not expensive, under $200.
2014 Palazzo 33.2
Cummins 6.7 ISB, Allison 2100 6 Speed