What you have with the phone and dash is called a "Ground Loop" Alas so long as you are DC all the way there is no easy way around it.
What I do is power the phone using an inverter and wall wart.. The transformer in the wall wart breaks the loop.
YOu can also get isolation transformers for the audio line but not many of them are ones I'd recommend (Heil Sound makes a good one) But when you go this route impedeance can be an issue
Better choice.. Connect via Blue Tooth or.. Charge up an aux battery (I have some good ones) and use it to power the phone..
ONe thing EVERY RVer needs is one of those portable 12 volt Jump Starters. at least 700 Amps peak.. Use the 12 volt outlet on it to power the phone.
Home was where I park it. but alas the.
2005 Damon Intruder 377 Alas declared a total loss
after a semi "nicked" it. Still have the radios
Kenwood TS-2000, ICOM ID-5100, ID-51A+2, ID-880 REF030C most times