Here is the parts breakdown.
Replacement parts for ROPER RT18AKXFN01 REFRIGERATORS | AppliancePartsPros.com

Here is a manual which should be close to your unit.
Whirlpool AM-4 Apartment Maintenance Series-Top Mount Refrigerators Service Manual - ApplianceDigest.com
You have to join the site to download the manual but it is free and this is a very good site for manuals and other info for the appliance repair DIYer.

Yours is closest to Section 4 (16 to 22 foot units).

Some units drain into the top of the fresh food section and then down to the drip tray ( Item 58 in Section 6) but I think yours goes directly from the drip pan (Item 7 in Section6) under the evaporator coils down to the drip tray under the unit.
Item 24 in Section 1 is the connector between the drip pan and the drain tube. The tube usually runs down the back outside of the fridge.

Sounds like drain tube is plugged so that the water from the defrost cycle cannot drain into the drain pan.

I would check that it is clear.
If plugged a turkey baster with warm water, an air compressor with low pressure or a wire can often be used to clear it.

Also check that it is not plugged with dust etc. at the drain pan end.
You might as well clean the condenser/compressor fan and the condenser coils while you are down there.

If it looks like stuff is growing in the tube I would flush it with a 10% bleach solution. This should kill the stuff and let you flush it out. You may have to do this several times, let it sit for 15 minutes or so between flushes.

Be careful that the drip tray does not overflow and empty the drip tral after you are done.

Be sure to unplug the unit while doing this.

You are welcome

And thanks for getting back to us. Now when others search for a similar problem they will see what actually worked instead of just suggestions about what could be the cause.