For one person a topper works great. For two it can be a little bit tight. Consider, for example, that unless you are able to stow all your gear underneath the sleeping platform, that gear is going to have to go somewhere at bedtime. The higher the sleeping platform the less headroom.
At least with a Silverado they'd have a bigger bed than in a mini truck. For a simple sleeping setup it can certainly work as long as the GF is OK with camping in primitive conditions.
The comment about lack of sanitary facilities makes no sense - if she needed a bathroom they wouldn't be camping in a tent either.
The key is not to think of it as an alternative to a motor home, think of it as a tent that has hard sides, doesn't have to be put up or taken down in the rain, and won't fall down in the middle of the night.
When I was single I spent many nights in the back of a mini truck (Ford Ranger, then Toyota Tacoma) and even though I'm 6'1", it worked for me. What I liked most about it was that it was SIMPLE.
Simple is good because simple means quick, easy setup and teardown of camp.
Wife and I have been camping in a trailer for the last 7 years. First it was a homebuilt teardrop, then a
T@B Clamshell, and now an R-Pod. Our trailer is roomy and comfortable, has a nice comfy bed, a 3 way fridge, heat in the winter and AC in the summer (as long as we have shore power) and even a bathroom.
But it absolutely restricts where we can go and how we camp, and I'm OK with that - all relationships require some compromise. But if I were single I'd be camping in a pickup with a shell.