I have a leak coming from inside the bell housing of my transaxle leaking onto my clutch making it slip new clutch replaced less than a week ago and rear main seal replaced oil smells like gear oil (rotten eggs) wondering where leak could be coming from? is there a seal behind the selector shaft?

oil or hydraulic fluid?
automatic or manual?
if manual, hydraulic clutch?

it is hard to diagnose with so many listed faults
first up slipping when hot indicates a weak pressure plate tension but the reasons o slipping when hot may start at a bad clutch master cylinder not fully allowing the brake fluid back to the reservoir ( blocked compensating port)
that creates a catch 22 situation where the pressure place is not fully engaged and so it slips marginally and that builds up friction heat which in tern cause what is called brake fade . In other words the friction surface ( clutch plate facing) cannot absorb any more heat and so it slips
moving on , the slippage can be from excessive oil on the linings but if that is the case it will slip or grab ( shudder) on take off
The oil will most likely be from from a front input shaft seal and that may be from failing oil seal or overfull transmission or blocked breather for the transmission
seal generally will produce a oil mist in the bell housing after time but adding to your statement of hard to get/change gears I suspect that the oil leak could be from a failing front shaft bearing of the transmission not allowing the seal to work properly
To sum up , the transmission will have to come out and the clutch kit and throwout bearing replaced
from the slippage I would recommend that the flywheel clutch face be machine to remove high hot spots and that combination will give a smooth clutch operation at this point if there are signs of a rear main seal leaking , replace it before the flywheel is refitted
remove the extension housing that the throw bearing slides on and the seal is in that housing
that front bearing can be checked also and if in doubt , overhaul the bearings in the transmission
(visible damage to the bearing cage will be evident here indicating a failing bearing)
when finished replace the clutch master cylinder if more than 5 years old since replacement
sorry not good news and it will amount to a costly exercise but there are no short cuts to a good reliable jib

the main leak will at the slave cylinder past the piston cup
if the slave cylinder is out side the bell housing the simply replace it with new one and bleed out the air
if the slave cylinder is inside the bell housing then unfortunately the gear box has to come out to fit the new slave cylinder

Clutch or automatic? The rear engine crank seal is most likely leaking. Oil on the clutch will ruin it completely, even the metal parts, so replace the whole thing. If it's an automatic, the oil is dripping on the flywheel which gets hot normally. No damage . Use a solvent to clean the oil from inside the bell housing after you replace the seal.

The clutch hydraulic system is very simple, just 3 pieces. There is the clutch master cylinder with attached fluid reservoir-mounted on the firewall, there is a steel line from the master going down to the transaxle bell housing, and there is the clutch slave cylinder on the bell housing that the steel line connects to.
If yours is leaking under the car, it is probably the slave cylinder, either that or the steel line. Either one should be easy to replace.

Do you have a hydraulic operated clutch? The slave cylinder for a hydraulic clutch would be on or sometimes inside the bell housing. Some ford and chevy trucks put the slave cylinder in a very difficult to reach place-the transmission has to be removed to service the slave cylinder. Most cars have the slave cylinder mounted on the outside of the bell housing. If they leak, replace or rebuild, although those internal slave cylinders I think can only be replaced.

The clutch master cylinder is located on the firewall in front of the clutch pedal and has a solid plastic line (hose) that runs directly to the clutch slave cylinder which is located inside the bell housing of the transmission. Unless it has rubbed against something and developed a leak, which is very unlikely, the leak will be at the slave cylinder and drip from the front edge of the bell housing where it meets the engine. This is a common problem with older Chevy trucks and the best fix is to replace the entire hydraulic system with a pre-assembled unit available at most parts suppliers. These units come with the master cylinder, hydraulic line and slave cylinder with a new release bearing (throw-out bearing) and are pre-bled and full of fluid. Replacing just the slave cylinder is a major job and bleeding these systems have been known to drive people crazy. Replacing ALL of it aint much more work and saves your sanity not to mention all those dirty words.

If there is no free play on the clutch pedal that mean the clutch disc have gone ,or adjust the clutch pedal.

If it's slipping it's near the end of its life and you'll have to change it. Make sure to inspect the rear main seal - most of the time a clutch slips because of wear but a rear main seal leak will allow oil to leak onto the disc and it'll slip. In either case you'll need a new clutch but if the seal is leaking it'll ruin the new one too.

valve cover is the most likely cause, if rear main seal were to leak it would not reach you clutch it would drain down and out the bell housing,now the clutch plate has to be replaced because oil is embedded inside and with heat it will resurface.while apart inspect the flywheel area really good for other possible causes.