Generally the max distance is around 90m, but play safe and keep it shorter.
Attached is a very rudimentary drawing of how it could be setup. Having the switch in the observatory allows you to have flexibility in the future if you want to extend the wifi signal using a wireless access point or add more networked equipment.
For the switch, you can use something like this one:
https://www.umart.com.au/product/tp-...l-sg1008-49451. You will need at least 5 ports if you plan on using the 4x Asiairs at the same time. Go bigger if you think you will need more ports.
Example wireless access point if you ever consider expanding the wifi coverage to the observatory:
https://www.umart.com.au/product/net...ss-point-61005. It will connect directly to the observatory switch. Will need a quick setup for Access Point (AP) mode, set the same wireless network name (SSID) and password as your wifi router in the van, so you can roam seamlessly.
Hope it helps.
Update: Uploaded a newer version of the diagram showing an optional WiFi Access Point if you prefer to connect the AsiAirs via WiFi or have other WiFi enabled equipment you would like to use. If you go the WiFi Access Point route, then you won't need the network cable between the switch and the AsiAir.