Twinboat has it right. 3 weeks parked is a pretty good length of time for an RV. You should expect 12 volt house power to be gone by then.
RV's frequently come with a disconnect switch near the entry door. Many people turn it "off" and come back in a couple of weeks and find the RV without 12 volt power. Some people call the switch a "salesmen switch" because it is the first thing a salesmen shows them when they first enter the RV.
Not every appliance is connected through the switch. The things still connected slowly deplete the battery.
Storing an RV:
Store connected to shore power or store with the battery disconnected at the terminals. In some RV's the salesmen switch must be "on" when connected to shore power to keep the battery charged.
Lead acid batteries must be stored fully charged
for long useful life. Fully charged means 14 to 18 hours on a high quality charger like the one in your RV. It does not mater what the starting state of charge is, it must be charged for a long time. If the battery has been abused, 24 hours or more of charging may be necessary to clean the sulfate off the plates.
Don't forget to check water level after charging flooded cell lead acid batteries. Charging consumes water.
Next, monitor the battery terminal voltage. Recharge 14 hours before the voltage drops below 12.4 volts.
Check frequently until you know how long your batteries will last.
, batteries may last 1 to 3 weeks
Parked with battery disconnected,
may last 6 months
Parked while connected
to shore power may last for years
. But, do check terminal voltage at first to verify things are working as planned. RV's are wired in many different ways. Many people have been surprised at how it behaves.
12 volt Side of Life part 1
The 12volt Side of Life (Part 1)
12 volt Side of Life part 2
The 12volt Side of Life Part 2
Some of the information is a little dated, but it is still pretty good.
How does the Lead Acid Battery Work?
How do battery chargers work
How to Charge and When to Charge?
I wish you good luck and happy trails ahead!