I've tried both, and both can be made to work.
Building a sun-house adds value to your home, and with glass everywhere and fitted out with a few plants you blur the boundaries between "inside" and "outside".
Problem is lighting and heating/cooling - things heat up and cool down fast, so spend on good double or triple glazing, and even then you'll need shades in summer.
Advantage is that you can tap into the services already in the house so no worries about cables (lightening!) etc. Plus if you need the can...it's right there.
I put a few extra bucks into the budget and made enough room for a meeting table which doubles as a dining area and is also perfect for Sunday brunch.
Buy dual-use wooden furniture, and make the tables into work surfaces by throwing leather hides onto them; cheap, easy to clean, decorative when reversed.
Someone else mentioned pets; yes, the cats love coming into the sun-house, but then of course insist on lying on the keyboard....ah, first world problems.
The RV ...you can pick them up for peanuts, then optimise for your use. Probably cost you less than a building a new office.
It's amazing how much space you get in even a small vehicle if you rip out the stuff you don't need; plus you can keep the can if you want...
Plug into your house power and you've even got ac and heating.
As a bonus, you can even drive it!