I had a FWC Grandby Camper for 8 years for exploring the back roads of the NW with my wife. We sold it to buy an old Airstream and put a truck shell (topper) on the truck. We did not take the Airstream down the gnarly back roads. We used the topper for carrying gear and at times sleeping when we did go down those back roads. We ended up really missing the FWC pop-up. So, we ended purchasing a FWC Shell. We decided to not purchase another FWC full camper set up due to cost, and also to keep the weight down. We leave the FWC on the truck all the time, even when towing the Airstream. I had FWC put some large closets in both rear corners and two small ones in each front corner. I then did some additional basic modifications on my own. We are very happy with the arrangement. As one of the other posters noted, one certainly appreciates the amenities campers can offer. I'm sure many on this forum have made the progression from tents to toppers to a pop-up camper over the years. While the full camper option certainly offers quick access to more amenities compared to the shell, we've found that it doesn't take that much longer to set up the inside once we get to our destination. And, as you noted, there's the cost savings. There's also the option to make some upgrades to the shell down the road if so inclined.
A year or two ago I posted some photos to the forum showing the very basic changes I made to my shell. And there are also several threads showing what others have done. I've seen some pretty impressive "do it yourself" shell builds on the forum and they are worth checking out.
Good Luck
TGK