Normal thread drift..
I looked at the idea of converting a cargo trailer, but really, they are even more expensive than a travel trailer of the same size.
They are much heavier empty weight than a travel trailer of same size.
Typically the smaller cargo trailers you would be considering are rather low head height unless you order one built to your specs.
HERE is just one example..
"
Standard Features
Ramp Door w/ Spring Assist (no extension flap)
White or Black metal exterior (.024)
12V Dome light with switch
ST205 15? RADIAL tires
Aerodynamic styling
Non-powered roof vent
Silver Mod Wheels
Aluminum fenders w/lights
LED Strip tail lights
32? Strap Hinge Side Door w/ Bar Lock
3/4? plywood floor/Painted Underneath
Wall members 24? o.c.
V-Nose with vertical ATP trim
3/8? OSB plywood sidewalls
Floor crossmembers 24?o.c.
1×1? steel tube in walls and ceiling
Roof members 24? o.c.
3500lb l/spring axles with 4? drop & Electric Brakes
75? Interior height
Screwed metal exterior
EZ Lube Hubs
2000 lb tongue jack
Galvalume roof
24? Stoneguard on front
2×4? steel tube main rails
7-way Round elec. Plug/ 2 5/16? coupler"
$4225.. for bare bones basic
To get what you want with extra height and extra options, you will have to go with the "premium version" At $5225 and the extra height is additional charge on top of that..
By the time you option a cargo trailer out your going to be easily $8K-$9K and that is before you even get started with making it a travel trailer..