These cargo compartments should be weathertight. I suspect even the model you saw had a latching mechanism, rubber gasket around the opening etc etc.
However, if it was NOT closed properly, sure water could get in.
On my Georgetown there are drain openings in the bottom of every compartment. I’ve driven and camped in all kinds of weather and (knock on wood) never had water intrusion. The compartments are made of a material (robo-cast) that aren’t affected by water, but eventually, what might YOU be putting in there that might not like getting wet?
Ken and Terry
2018 Sunseeker 2430S-CD over a 2017 Ford E450
and if we ever need it, can be pushed by our 2017 GMC Terrain toad (4 wheels down)
Former Georgetown 330TS owner for 10 years with more mods than I can count.