Yes we have them  under a custom made mattress. We started out many years ago doing the bed quest for better sleeping. Adding different types of foam to the tops of the cushions. Then doing the froli springs. Finally we decided to bite the bullet and do a custom made mattress, That was the best investment. So now we use the from springs under the mattress to keep it up off the fiberglass so it does not sweat. 

Randy,
 
Just wondering where did you get your mattress?  I was informed by the boss we need to look into improving the sleeping area (aka the mattress).
 

I would not mind paying Catalina Direct the extra $40 to support them, over the normal US distributor, Nickle Atlantic:
https://froli-systems.myshopify.com/collections/travel/products/froli-tr...

For those of you that have installed a Froli, how did you handle the cutout / removable part of the v-berth?  Did you cut it open and install a Froli spring underneath, or just ignore the 1.4" difference in height?

We got our mattress from ​riversidemattressinc.com 
As for our cut out I added a piece of 3/4 inch plywood to replace the the cushion then used the from springs.  The mattress we got  is one piece. 

I installed the Froli system over my old original cushions and I felt it made a big difference.  I dont regret purchase!!