If you want to run on a separate tank, simply remove the gas line input to the mechanical pump. put the two 5 gallon tanks on the front bumper and hold in place with bungie or ratchet straps. Run a 3/8" gasoline hose from the input side of the pump to one of the gas cans and away you go. Stop every 40 miles or so and switch tanks before you run one dry.

I had one once where the mechanical pump was removed. On that one we put four 5 gallon gas cans on the roof and ran a 3/8" ID hose down the side of the coach, we disconnected the selector valve and connected that hose it to the output hose that was on the selector valve. No pump was necessary as the gas would gravity feed the system. It was a real pain this way climbing out on the roof every 40 miles but we did manage to keep all of the gasoline outside rather than inside with us. Having been through one coach fire I am terrified having gasoline inside with the passengers. We ran a little over 800 miles that way and only ran one 5 gallon can completely dry.

One hint, If you do this, get a 1 foot or so piece of 3/8" OD steel pipe or steel / copper line and stick it on the end of the hose going into the gas can. That will keep the pick up end on the bottom of the can. We also stuck a short 1x2 under the can to tilt it towards the pick up end of the can. We got maximum range per can this way.

Good Luck.