I've faced this same problem in both our Airstream Class A and Forest River Class C rigs. Bluntly, there is no ideal solution. You will be compromising on power source versatility or features. Sound quality can be dramatically improved, but you shouldn't expect a home audio experience, either.
You are already aware that stuff supplied with and designed for RV use is NG in any respect. The speakers are poorly designed, the electronics are poor quality, sound quality is generally awful and even convenience is often compromised by feature load and/or price point.
Given that reality, you'll have to start by deciding if you want to adapt your dash unit or the entertainment system. Both require compromises and a fair amount of DIY handiness with carpentry and basic automotive electrics.
The dash unit has better low-cost, high-quality alternatives because you can find decent head units either powered or unpowered that can take BT input and have CD spinners at many salvage yards and pawn shops. Alpine, JVC, Panasonic and Sony all made several good ones and each have a few still in current production as well. If you can find an unpowered unit, you can set it up to run one amp for driving off the front stereo output and a second off the rear that you can use to plumb exterior speaker connections for camping. That gives you the most options in head units, power amps and speaker pairings and is also the most flexible for getting good quality sound outside. Runs on the engine battery though, so you have to check if you have an emergency start circuit to crank the engine from the coach batteries if you run the start battery down.
You could also consider going to the McIntosh or Nakamichi autosound systems, but that would take some serious searching and be difficult to repair if a problem cropped up. The Mc was made for the circa 2005 Ford GT only, and replacements are no longer produced. Audio Classics occasionally comes up with some bits, but you'll more likely have to wait until somebody wrecks their GT. Could be a long wait. Nakamichi's system isn't quite as rare, but has been out of production longer and can be very difficult to service. Same issue with the start battery for both.
Going with an entertainment system solution, you have to decide if you want surround and where you want to distribute the sound both internally and externally. Also, you will have to run off an inverter when dry camping unless you can tolerate generator noise while listening (I can't, but you have to decide for yourself). The AC / inverted DC solution allows you to use just about anything you like within your wattage limits, so you have a lot of choices.
Wiring the entertainment system is a serious PITA, so look at that before you do anything else when considering this avenue. Almost certainly, your rig has built-in audio and video cabling in the walls that can't be changed. You likely also have very limited space to install a solution. You could go with one of the compact lifestyle systems out there, but then you have to figure where to put the speakers, how to cable them and note many of the new lifestyle systems don't spin discs. They rely on BT and wireless instead. Alternatively, you could go with a compact AV receiver, an all-in-one disc player and just upgrade the built-in speakers. Note that getting surround sound can be difficult, if not impossible because of the wiring limitations. If surround can't be had, you can go with a much simpler stereo receiver with BT and just live with stereo-only video.
So you know, we started our RV audio adventures in 1989 and have yet to find a really good aftermarket DIY solution at any price. Companies like Creative Mobile Interiors in Columbus OH make a very nice living doing just this kind of work, but you really have to love your rig to drop that kind of coin. We thought about that for our Airstream, but just couldn't justify it in a 17 year old MH on the P32 Workhorse chassis
The compromise we finally decided on was just living with whatever crap the RV maker provided for the coach entertainment system and upgraded the dash unit for our driving pleasure. When we go outside, we have a really good boombox with CD and auxiliary input so we can stream from our phones. (yes, Panasonic and Sony still make a couple of nice ones). The box runs on dry cells or a low wattage inverter from the coach batteries. Simple, inexpensive and if the box gets clipped or left in the rain, no serious harm done. It won't compete against the yahoos with DJ systems, but it also won't land the park minders on our case either.
Hope this helps give you a starting point & let us know what you end up doing. Happy camping and listening!
