If the tank is full it will remain full unless you drain the entire system (like preparing for winter/weatherization...) City water pressurizes the whole fresh water system of the trailer, including the hot water paths through the heater and to each hot tap. If you do not get any air when you open the tap, the system and hence the heater is full. When camping away of park hook-ups, you turn on the electric demand pump and it will run intermittently to keep the water system at a nominal pressure, both old and warm pathways. I never drained the system during the active season, only during winter when I had a small travel trailer. For my large diesel motor home, I lived in it full time and therefore never drained it since it remained occupied and heated year round.
As to your table, if it has a single or dual central column support, it should just lift off. They can be tight when new. Another type of table has a folding leg and a hinge/bale mechanism at the wall that allows it to be lowered into the support height for the bed conversion. Maybe you can post a picture of which type you have...
Here is a representative video not specific to your trailer.
https://www.google.com/url?sa=t&rct=..._20U0_Dgd_uUeM
...and here is the 'bale' type mechanism...
Chuck