The blue tap at base of tank looks like drain to empty tank.. You only show one pipe coming out of it, should be two one to fill another attached to internal pump.
The yellow tap is drain for system and is show open ie up and draining.
Here is inlet and filter that should go into hole where blue hose is.
Crystal Filtapac Replacement Water Filter Housing
www.ukcaravans.co.nz
Should be two 12V electrical connectors on back of it to power external aquaroll pump.
There should also be pressure switch in system. See link. It fits between inlet and internal cold water pipe. Pump's +12V goes through this switch. Turns pump off when system is at correct pressure. Knob on top allows you to adjust cutoff pressure.
The Whale MK2 Inline Pressure Switch (12mm) allows regulation of the pressure in your water system.
www.rvsupercentre.co.nz
I'd get it working of external aquaroll pump first. Then you can think about plumbing in internal tank and way to fill it from aquaroll.
I'd feed internal tanks outlet through pressure switch and have pressure switch control internal tanks pump.
To fill tank feed external inlet pipe straight into internal tank inlet. Connect 12v to external inlet pump connectors. No need for over flow if you fill from 10L containers ie place aquaroll pump in 10L containers. Only add what you think internal tank can take ie 10L at time. Error on caution side and under fill.
My caravan is feed externally using same inlet. Plan is fit internal tank and fill from Aquaroll using external pump. Not sure if I'll use inline pump of submerged pump to supply water from internal tank.
Can you supply photos of pump in internal tank, how it is mounted, plumbed and wired. I may copy it.