Your "box within a box" design is perfect for the location, it prevents interference with the drawer from other stuff in that storage area.

I'll definitely be "stealing" your design! :-)

As far as the fire extinguisher is concerned, I found the perfect spot right beside the door under the entertainment unit, first panel is unused and just wide enough to hold the extinguisher.  I moved it there last year in prep for this drawer.

Again, nice job!

In our 2018 178 HRE, the dinette seating is on the front end of the trailer. We did not like removing the cushions and lifting the plywood to access storage. There was always something on the seat and going to outside access at night, or in the rain did not work for us. And, we desired more clothes storage. So I set out to build some drawers for under the dinette seats. 

I figure I could size them to get good value without compromising the outside opening, inhibiting loading of the front storage. The drawer by the frig will be a bit narrower to address the under frig storage and power outlet. I will have to remount the fire extinguisher to under the entertainment center. There is a 110V outlet that needs to be moved. I found that by rotating the wood panel with the existing outlet hole will work to move the outlet.

 

After considering the above, I drew up some plans to build the drawer out of a 1/2" birch plywood, 2' x 4' project boards I found at Home Depot. These boards are nice and smooth and need no sanding. I used 3 of these boards for the two drawers. I used Liberty drawer slide hardware which seems to be what FR used for the kitchen drawers. I used 5/8" plywood, about half of a 4'x8' sheet I had laying around. Total costs, about $100.

 

Using a putty knife, I easily popped off the corner trim and wood panel without damage to expose the opening. Save the wood panels for later to cut the opening for the drawer and remounting.

To hold and support the drawer, I built a supporting box 1 inch larger than the drawer. Drawer slides need 1/2 space each on the side of the drawer. Using a table saw, I cut the drawer and box pieces. Then, drilled holes and used woods screws and a bit of wood glue to assemble things. Installing drawer slides 1/8" above bottom of the support box bottom made a nice efficient fit. To the front drawer box, I added another piece of birch plywood cut 1/2" larger on all dimensions of drawer to create a front panel to hold the pull and close onto the wood panel. I did some light sanding on all the edges. I added 1x2s to the bottom of the support box for alignment with the dinette frame and screwed the support box into the seat frame and floor. To finish, I added cabinet door catches in the back to keep the drawers from opening when travelling and stained the front panel with a custom stain. (described in another forum thread)

Below are some pics. I've listed piece dimensions for one of the drawers at the bottom. I found this project fun and a bit challenging to design, but very satisfying with the new storage utility added to my R-pod.  

Supporting box

Drawer

Drawer without front face board

Support box sitting on 1x2 seat frame

At the back of drawer

Other drawer: 9"x23, for a 10" opening

unstained face boards with pulls installed

Drawers shown partially open. Slides allow full drawer to be open.

Finished!

Drawer pieces of drawer nearest to door - all dimensions in inches.

Drawer: 2 sides 10x18, front and back 10x16, bottom 16x17, Front face board 19x12, opening in wood panelling 11x18
Support box: 2 sides 10x19, back 10x9-1/4, bottom 18x18

Used #6 wood screws 1" for drawer and 1 5/8 screws fobr support box assembly
Drawer slides: 16" for the drawer by the door, 22" for the other.   Amazon: Liberty D80616C-ZP-W 16-Inch Ball Bearing Drawer Slides

Design notes - for visual