Yes, there is an option for you to use the much higher quality tankless water heater as well as a door that is going to work for your 15-1/2" x 16" cut out. What you will need for the water heater itself is the Furrion unit you mentioned:
- Furrion RV Tankless Water Heater - Gas - Automatic Pilot - 60,000 BTU # FR93FR
Then for an access door, if you take a look at the part of the instructions I added, you can see which doors fit specific cut out sizes, and since your cut out is 15-1/2"... view full answer...