There are all kinds of “stand alone” tents on the markets in all shapes and sizes. You don’t even need a minivan camper conversion to camp with them. They can be anything from a back-packer’s sleeping tent, to a huge two or three room (or more) tent with one room for both screened room and privacy. These would be great as additional sleeping quarters if you were traveling with more than just one or two people. But they have no “connection” to the minivan camper, so they remain a totally separate unit.
Everyone has different needs, and maybe there are times when something like this would be handy. Only the reader can make up their own minds. Two key features to keep in mind on tents of any kind are (1) a rain flap, and (2) an awning.
A rain flap is that extra little roof that you see over so many tents, and it serves two purposes:
(1) If you have sun beating down on you, it allows the suns rays to be absorbed by that, rather than heating the roof of the tent and making it very hot on the inside. The air space between it and the roof allows for circulation to carry some of that heat away and keep it from being transferred to the roof of the tent.
(2) When it actually does rain, it keeps it from saturating the roof of the tent and possibly causing a leak. Many of the older tents made of canvas created their “waterproofness” by trapping small air bubbles. I remember as a Boy Scout being told “Don’t touch the tent when it’s wet, or it will start leaking”. The simple act of wiping a finger across the fabric would break the air seal and allow water to soak through. Many newer tents use much improved fabrics and even a process and chemicals similar to “Scotchguard” to make their tent fabrics less vulnerable to leaks. It causes the water to bead up and run off, rather than emulsify and soak through.
The other thing that you may want, especially if you don’t have one on your vehicle, is a nice sized awning. Many times during the day, it will be too warm inside the tent to spend much time there, and sitting outside in the sun is no fun either. You’ll want some shade, sooner or later. And if it’s one of those days where there’s just a heavy mist in the air that never quite turns into a full rain, it’s nice to be able to sit outside without getting wet. But a little 3 x 4 awning isn’t going to do the job, either. You probably will want something about 6 x 8 or even larger if you can get it.
If the tent doesn’t have it, then you might want to think about a “quick to set up” portable canopy, although they may be too bulky to be standard equipment in your minivan camper. Only you can make up your mind what’s important to you.