Replacing TT radio with used car radio ???

1) No, most auto head units have their own amplifier.
2)Maybe, but not typical.

Look around at Crutchfield to see what's available and for installation instructions. You can use different bezels to accommodate different sizes of radios.

Go to walmart and buy a $50 car radio. Or splurge and get a nicer one. Even check out Crutchfield (do a search, they have a ton of stuff).
Almost all have 40-50w of power power channel and will not require an external amplifier.

Buy a $100 new radio from Crutchfield. It will probably have a USB slot for mp3's. I use a $10 NanoUsb for music. If stolen I'm out $10. Plus the new radio will work. Crutchfield will usually throw in the wiring harness and dash plate.

X2 on walmart.

They have a 'huge' selection of radios and really cheap even for some pretty nice pricey brand names.

Sound system store wanted $800 bucks to replace the radio in my MH. I picked up a $120.00 radio for $79 bucks on sale that sounds way better than the factory one did and has 4 times the options on it. Not to mention some totally unecessary but cool blue lights.:B Instructions included were easy enough for me to install too!

Depending on how old the tech of the failed radio is, you may find you have to do a little extra wiring to keep the Time and Station Memory "warm" when you turn the TT's house battery off for storage.

Any way I look at it, I see this swap as "easy."

I replaced my OEM radio with a Panasonic, I think, and the only problem I have is that the wiring from the radio for the various options like Fade, Left/Right speakers, etc. were never connected properly so we just use the radio with the speakers that are connected.

In a class A often there is an amplifier, but if you swap out the radio the amplifier will remain since the dash radio is a standard car radio.

in a trailer, Often all the radios are standard car radios to start with, Nothing special about them, some are DIN, some Double DIN some 1.5 DIN (DIN is the French national brueau of standards and the 1, 1.5, 2 refer to the size of the cut out which as you said you can re-do) so with very few exceptions they all replace each other.. and with the DIN cutout radios it's very easy to swap 'em out.

You may with to get an Adapter cable (www.crutchfield.com should have 'em) if the radio is a different make from the one you have.. Saves cutting and splicing wires.. Personally.. I'd cut and splice, but then I have the tools and training.

Ditto Crutchfield, I just put a Kenwood out of my truck (aftermarket) into my RV but the speakers that are in it are pretty low end, I have much better speakers but they are too thick to fit in the roof holes, I could put some spacers and make them work but the slide would then hit one of them so I'm out of luck unless I mount them somewhere else but then would have to run wires which is a major pain if even doable. I would personally just get a low end modern unit maybe with Bluetooth and a usb for charging phones etc and be done with it.

Make sure whatever you choose is rated for your rig's speaker IMPEDENCE which is stamped on the rear of the speaker usually on top of the magnet structure.

Mis-matched components will make your setup sound like a Mexican Hi-Fi

It sounds like the OP wants to pull an OEM (stock) radio out of a junk yard, which in theory would work,but those usually have proprietary plugs and you'll have a heck of a time trying to figure out what wires go where with no guidance. I'm with the others suggesting a radio from Wal Mart.