There are THREE drains under your trailer. Two (usually close together) are the "low point drains" for the hot and cold water lines". The third one will be under the fresh water tank and will have either a cap or a valve on it. It is the fresh water tank drain.
As for your slides, it's hard to say whether not using them for a couple years would damage the mechanical components (motor, ram, gear sets, cables, pulleys, etc) but it's a "sure bet" that the rubber seals have become brittle (not easily bendable) and possibly adhered to the sidewalls and top of the slide. If the slides are in the travel position (inside the RV) you might want to go inside and use a paint stir stick or a similar device to push the seals away from the sidewall to make sure they aren't "stuck to the sides". While doing that, check any cables, pulleys and wiring to the motor for any visible damage, corrosion or other signs of failure.
Make sure you have a fully charged battery in the trailer before attempting to move the slides. A partially charged or defective battery will cause the motor to overheat and possibly do more harm than good.
2015 F250 6.7l 4x4
2014 Cougar X Lite 27RKS