Since you are switching to a Furrion water heater you will need to replace your water heater door as the vents will not line up.
As I can not determine the size of your opening from the images we have either the Furrion RV Tankless Water Heater 16" x 16" Door # FR98SR or 18" x 18" Door # FR68SR. Both water heaters will require a space that is 12-5/8" wide x 12-5/8" tall x 19-1/4" deep. view full answer...