The new trailer skin for the Cabin is too good to be locked in multiplayer jail, so this mod allows Robin to build you one on your farm as a separate building!

The small Camper building, with an interior size of 11x6 (66 squares), costs:
7,000 g
15 Refined Quartz
5 Iron Bar
1 Battery Pack

But it can also be upgraded to the Deluxe Camper for a larger interior of 20x9 (180 squares).  This will run you:
12,000 g
25 Refined Quartz
3 Fire Quartz
1 Gold Bar

The Deluxe Camper comes with two options for the interior, the standard version, an empty room which can be used like a shed, and a greenhouse version, with a small, 5x5 patch of tillable ground on the right side.  You can choose which version you want in the config file that generates in the folder the first time you open the game with this mod installed.  Look for the option "DeluxeInterior"; your options are "DeluxeVanilla" or "DeluxeGreenhouse".  By default, this is set to DeluxeVanilla.  You can still change the flooring of the CustomGreenhouse, it will simply skip over the tillable ground and its border tiles.

The exterior is 5x3, making the whole thing a smaller, cheaper alternative to the standard Shed and Big Shed.  Machines and furniture can all be placed inside and the floors and wallpaper can be changed.  And if that wasn't enough, both the Camper and Deluxe Camper come in a variety of skins:

Vanilla (credit to ConcernedApe)

Banana Split

Espresso

Mint

Strawberries and Cream

and Neapolitan


Be aware: You can choose the skin in Robin's menu when building the Camper and it will be built with that skin active, however building the upgrade will revert it to Vanilla even if you choose an alternate skin in the build menu.  You can still go back to Robin once its built and change select a new skin at any time using the Paint Building and Change Appearance options.
I would also like to add a function where it re-centers any items mounted on the back wall during the upgrade, but at the moment the IndoorItemMoves function appears to only apply to objects not to furniture.  The back wall remains in the same location on both interior maps, so items placed on the wall before upgrading will stay where they are on the wall, the wall just extends further to the right after the upgrade.
I tried my best to add a PaintMask and tried a dozen different ways but never could get it to work.  I've included the PaintMask file if anyone wants to try adding it themselves.  If you succeed, let me know how and I can try adding it into this release.

The vanilla design of the Camper and Deluxe Camper are in the base game (I removed the mailbox and front porch area), but the skins were all designed and illustrated by me.
Also, try checking the left side of the building exterior for a fun surprise!

Screenshots seen here are taken on my modded Cozy Desert Farm.

Version History
Ver 1.0: Camper introduced
Ver 1.1: The Deluxe Greenhouse interior, which can be selected through a config file, introduced as an alternative interior layout to the standard interior.  The default wallpaper and flooring changed to better suit a camper interior; note, this only affects Campers built after ver 1.1 is installed; Campers built while this mod is still on ver 1.0 will retain their original default wallpaper and flooring.  Thank you for the suggestions, guys!

Translation Offers Accepted
If you'd like translations for this mod, I am working on adding i18n compatibility, but what I lack are the translations themselves.  If anyone would like to contribute translations, I'd happily add them in and give you full credit.  What I'm looking for is a translation for: "Camper" and "Deluxe Camper"; a set of five names for the alternate skins (I used ice cream flavors, but I'm down with whatever kind of localization contributors think would work); and the descriptions for Robin's shop, "A small camper you can use on your farm for extra space.  The interior can be decorated." and "Makes your camper bigger.  The interior can be decorated."

Thank you to Sesi5 for the Italian translation!