I would want to avoid taking that nice clean digitally-sourced sound signal and pushing it through an analog FM radio set. I understand doing that in the truck, to take advantage of the nicely built-in sound system, but in the "clean sheet" environment of the trailer there must be a better way.
The XM portable box presumably supplies power, antenna, amplifier and speakers. If a separate antenna is discreetly installed in the trailer, and a 12VDC power wire is similarly built in, that leaves the requirement for an amp and speakers... or an amplified speaker set, as used for computers. They could be mounted to walls or cabinets as appropriate.
As long as the trailer is not insulated with that foil-covered bubble wrap - and maybe even if it is - the antenna should work on the inside of the trailer shell... but the correct side will still likely need to face up. Try that in an Airstream!
There are many dock/amp/speaker rigs for portable music players, usually labeled as intended for iPods, which would work as well if there is a reasonable place to sit the XM receiver, and separately wired connections (not just an iPod dock connector). These would work fine, but I don't see them having much (if any) advantage over the real XM portable box; if they are really small, they won't have decent speakers.
1979 Boler B1700RGH, pulled by 2004 Toyota Sienna LE 2WD
Information is good. Lack of information is not so good, but misinformation is much worse. Check facts, and apply common sense liberally.
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