I added the 100 lbs tank by adding a tee at the point the right side tank went into the propane system. Photo attached shows what I did. Working great so far. The existing pigtail is connected to the right of the tee and the new regulator and extension hose are on the left. The hose runs behind the 30 lbs tank in the bay to outside the RV and connects to the 100 lbs tank, which sits under the 5th wheel area. When in transit the hose rolls up and I zip tie it to one of the holes in the handle of the 30 lbs tank. The hose and extra regulator were the expensive parts, but overall the total cost was about $60. A link where I bought the regulator and hose are included below.
Parts list:
1 - 1/4" pipe thread black galvanized tee
2 - 1/4" pipe thread black galvanized coupler
1 - 3/8" flare union
3 - 1/4" pipe thread to 3/8" flare adapter
1 - HP regulator
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
1 - 12' propane extension hose w/ 3/8" flare fittings
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
and a bunch of telfon tape and pipe compound to make sure it doesn't leak!
Apologies for the orientation of the photo... I tried 10 times to get it correct but no luck. The two red regulators are the top of the pic.