Even when just using fresh water tank/pump it needs refilled from a water supply source.
That source can have debris/sand/dirt in it.....plus when heated minerals can be left behind
So YES...filter water when filling fresh water tank.
Even with just an in-line carbon filter (blue one that connects to hose) is better then no filtering.
Although it will have no effect on scaling from heating the water. That would take a lot more filtering
Just do a good 'routine' drain/flushing of water heater to minimize scale build up
I ONLY use the fresh water tank/pump regardless of where/how long we stay-----in-line carbon filter for refilling