To remove the fuel tank you have to remove the rear guards.
The remove the rear guards you have to remove the seat and the steering wheel the battery mount and raise the control turret about 3" then rear guard will slide out backwards.
It is a lot less work to drop the tranny out .
To inspect it put the mower on some stands and use a long arm inspection mirror.
The belts have a bad habbit of slipping off the pulley and running between the pulley and the fan.
The pulley is either keyed or splined onto the input shaft depending which tranny was used.
The tranny is held in by 6 bolts
Two on each axel and two more at the front to prevent it rolling over under load.
One of the techs here put me onto this a long time ago.
The anti roll bolts regularly fall out and in some cases shear off.
The allows the tranny to twist or tilt forward , the belt then becomes loose enough to slip.
The other common problem is debris building up under the brake pedal stop which prevents the brake pedal returning all the way.
As the brake & clutch work off the same pedal, if it does not return fully the clutch does not fully engage and again the belt slips.
Finally, what if anything have you done to the mower.
Very common for owners to pull the rear wheels off and the drive key fall out unnoticed .
With out it the mower starts to move a little then the axel spins inside the wheel & you go nowhere.