I strongly recommend never running electronics on a battery charger. They put out a lot of trash. I know some have a power supply mode but why bother? A good 30amp regulated power supply (with clean output) is just a few dollars more than this Walmart offering and can be used for everything you need (battery connected or not).
And no, having your ship battery in parallel does not act as a filter for the charger trash. This trash is AC ripple that rides on top of the DC voltage.
This one is what I use and recommend (but there are many options once your look beyond auto part stores):
https://www.dxengineering.com/parts/mfj-4230mv
Consider this simply a replacement for your alternator for hangar flying. It will run you panel and charge you battery(s) at the same time. I run it to a pig tail with connector I pull down from the bottom of the panel, the power getting to the ship via a “Shore Power” switch breaker (15A-20A will work fine). For those who take the step to make the panel removable this is the supply to run the panel on the bench.
For you pMag users remember powering up your ignition(s) with a battery charger connected is forbidden.
Carl