I've been looking into the same thing for a 2017 RAM 1500. From everything I've been able find out, it appears that auxiliary tanks for gasoline have been outlawed. There are lots of options for diesel, but nothing for gasoline. There are a few options for transfer tanks. The difference being that a transfer tank sits in the bed and holds fuel, but is not plumbed into the fuel system or fed into the stock tank. You would have to pull over, get out, and pump the fuel into the onboard tank. I've found titan has
15 Gallon saddle tank but it doesn't have a built in pump only siphon. I've also seen combination toolbox / transfer tanks with built in pumps in the 50+ gallon range.
I have a 26 gallon tank and the only option I've come up with for a larger plumbed in tank is, as others have suggested, to replace the stock tank with a larger tank if it was an option or there is an aftermarket tank available. I followed the link enblethen posted above and it appears the tanks from summit are only for diesel as well. I haven't seen an aftermarket larger tank for the RAM. The OEM XL tank is 33 gallons and if you can find a dealer willing to quote it, most won't, the consensus seems to be it will be in the $3-4K range. Not worth it to me for 7 gallons.
With respect to others advice of just stopping, its not always that easy. It's a chore to get into and out of even the large Travel Stops with a 37' trailer in tow. I start looking to fill up around 1/4 tank and most times its not a problem, but on at least one occasion I have had to resort to finding a spot to unhook the trailer because I was running on fumes and there wasn't a gas station I could get the trailer into. I tend to stop and stretch my legs every couple of hours, but I prefer to do that where there's plenty of room to park. There are also still a few areas of the country and especially the trans Canada highway where I would definitely want more range than my stock tank offers.
2018 Forrest River Salem Hemisphere 282RK - 2017 RAM 1500 TV
Previous RVs and TOADS
2004 Fleetwood Bounder 32W on WH W20
2000 Four Winds 5000 21RB
1986 27' Allegro
TOADS
2005 Ford Ranger XLT 2WD
2004 Suzuki Aerio
1988 Chevrolet Sprint