Roy, I work for a tank distributor and see a lot. Gravity is bad news, and may be illegal in some areas. If a hose or connection gets a leak, you lose the tank load of fuel on the road. A fuel pump is better, but how to you know when to shut the pump off? We have a mechanic at work that ran his over three times, once in our gravel parking lot at work, once at home in his drivway, and once at a "Home-Depot" parking lot! That brought out the police and fire department!
If you do use a small electric pump, tee into the 2" filler pipe on your main tank, also add a second tee to vent both tanks together so they are common, plus you must also vent your aux tank to atmophere.
I run a "Transfer-Flow" 98 gallon tank, which is automatic transfer of fuel from the aux to the main tank. I have a read out in my cab that shows when the pump is on, how many gallons are in each tank, plus total gallons of both. This gives me a total of 132 gallons and allows me to shop for fuel a little. I save about 40 cents a gallon between my Home State and where my Daughter lives, so when we visit we fill up.
I urge caution with you installation because of what one of my co-workers experienced above.
Wish you well, and hope this information helps.
Tom McCauley (DP Member #513)
"Tankers-ToyII" Loaded 05 D/A K-3500 CC, SRW. Deep pan on Allison w/Transyn. Edge Juice w/attitude, 4" Kennedy exhaust, 98 Gal. Transfer-Flow cross bed fuel tank, Leer 100XQ bed cap. Reese 14,000lb class V hitch w/ dual cam HP sway control. Tow 34ft. 32FKD Holiday Rambler travel trailer. GCW 20,360lbs