Okay... first question... where in the northeast are you heading? If you are up this way(New England), I would be more than happy to help you out... though, the summer is my busy season....
Second....
No, a forklift tank holder will NOT work for a 20 lb bottle....
BUT... you CAN get 33 lb cylinders... and get them in aluminum(BETTER OPTION). I would order it with a vapor valve, instead of a liquid valve. That way, you have secure mounting options available off the shelf, AND, you can use it either vertical, OR horizontal. They also come with the 1 3/4 propane filler, SO, you are not just limited to the filling stations... but, you can stop at ANY propane bulk plant, and they can fill it, OR, you can be filled from a bobtail.... Just food for thought(I used to work in propane, and know more about it than I want to, so, any questions there, ask). there are also vehicle mounted tanks that can be filled remotely.
Mr Beast, I know you have fab background, but, with that much mounting on the back, I would recommend a double shear on the mounting arm, vice the trailer spindle, double shear may be overkill, depending on the intended use, BUT, having broken one myself....... Just my 2 cents....
You have a great starting point for a build..... take your time, and do it right... there will be things that change along the way.... I know you have a plan... and that is great.... but, I can tell you, other than some track cars, and race bikes... ALL of my builds pretty much evolve, and take their own direction.... And I think you will find, most of us do that.....
Start with your basic footprint and plan... and let it evolve... That is the best advice I can give you.....
What are you planning for the propane(I tried to keep up with the thread, but, it was off track, and I stopped checking)? Are you going to mount cooking appliances TO the van? Heat., hot water, etc? Or, just portable bottles, for your camp stove and grill? Any thought into a "toilet"?
Nothing here is meant in ANY other way, than question, and info... take what you want, and leave the rest, I wont get offended.
Chase