You need at least a 3 in. pipe -- shower is 2 in. pipe
If you reduce the soil connection to 2" and install a trap
The drain from the toilet to the main drain is plugged and the branch to the shower is not. If the main floor toilet flushes, it is between the two toilets and not between the house and the sewer.
The drains for the shower, toilet and sink all connect to a common line. Assuming the toilet is not backing up, the problem indicates that there may be a partial blockage, enough so that some of the toilet flow is backing up through the shower drain line. You need plumbing repair. The shower leak you mention should only be in the shower, or else you have a broken drain line as well.
If you have a drain available. You may need to chip some concrete around the drain to install the bottom half of the drain. With that in place, set the shower over it and the top piece screws into the drain to form a seal. You can build a 2x4 frame and raise the shower up enough to run a drain to a sump or a floor drain.
Grey is usually dirty water but not toilet water (shower, sink drain) Black (or Brown) is sewage from the toilet. If you have a motorhome there will be two sewage tanks... "grey" and "black"
If you reduce the soil connection to 2" and install a trap
A shower drain is 1 1/2 or 2 inch and a toilet needs to be either 3 or 4 inch.
I can be but the toilet needs to have 24" in front of it to be legal. Plus you cannot tie your shower drain in the the horizontal branch of the toilet.
This can be a complicated job as the toilet drain ranges from 3" -4" and a variety of materials from Plastic to cast iron to galvanized. Then there is the venting to consider and NO a shower drain CANNOT be used for a toilet discharge as the shower drain is normally 2" diameter
The drain from the toilet to the main drain is plugged and the branch to the shower is not. If the main floor toilet flushes, it is between the two toilets and not between the house and the sewer.
A down line clog would cause this. The shower drain is lower than any other drain in the bathroom so it is the first to tell you of a clog in the line. Time to Roto Rooter! Y-THINK-Y The toilet and shower must have the same drain. There is a blockage somewhere after the toilet and shower drains meet. The backup takes the easiest route back which must be the shower drain. Try snaking out both drains. Start with the toilet. To do the best job, remove the toilet and start from floor level.
The toilet is 3 or 4 inch and the shower is 1 1/2 or 2 inch. You could reduce the toilet to 2 inch, but it would never flush correctly or be of much use. The current toilet drain has no trap in that section because one isn't needed. The toilet itself is the trap. If you are using existing waste lines, they should already be connected to the vent stack. If you are adding lines, connecting to the existing stack is fine as long as it is above the last drain.
On a standard toilet in America the drain is 12 inches from the back wall to the center of the drain.
Your drain line is plugged somewhere after your shower and toilet line are connected. The reason for the gurgle in your toilet is because air is trapped in the drain and your toilet is the closest place for air to flow to. If you had a vent on your toilet trap arm then it would gurgle in the vent rather in your toilet but your drain is still plugged and needs to be snaked out and maybe your drain line needs to be examined for right grade or broken.
The drains for the shower, toilet and sink all connect to a common line. Assuming the toilet is not backing up, the problem indicates that there may be a partial blockage, enough so that some of the toilet flow is backing up through the shower drain line. You need plumbing repair. The shower leak you mention should only be in the shower, or else you have a broken drain line as well.
If the drain is adequate size, 3inch, then you could.
If you have a drain available. You may need to chip some concrete around the drain to install the bottom half of the drain. With that in place, set the shower over it and the top piece screws into the drain to form a seal. You can build a 2x4 frame and raise the shower up enough to run a drain to a sump or a floor drain.