Not sure if this would be of any help to you.
BLT Offroad Avalanche
The Avalanche is similar to the Suburban undercarriage. Mine is a newer model, however there may be some similarities?
I removed the spare tire and installed an 18 gallon fuel tank. It tucks up nice underneath.
I decided to carry my sand ladders under it They also act as a skid plate
Two filler necks can confuse the attendant some times
This has been working great for me all these many years now. No regrets. I found the extra 18 is just about right. I have a comfortable range around 500 miles. I can push it to 700 if needed. More means more weight. I did that on my last truck where I packed 35 gallons over the stock fuel tank. I rarely dipped into the third tank. For me 20 gallons more than stock is plenty. My Avalanche has a combined capacity of 49 gallons.