Also, can I use the accessory terminal on the GMPP fuse box ?? That is protected with a 50 amp fuse. Or should I just use the Batt lug terminal ??
It depends on what you are powering off of that terminal and if what is wired to that terminal are on or off.
On the above drawing you have a stud that is an accessory output and is fused at 50A.
If one were to draw 30A off of that terminal and it is for powering the Vintage Air, then technically this leaves an additional draw of 20A or less on that terminal that one could use for something else. So if you have a 30A draw, this taken away from 50A leaves 20A.
Now with that said, you could have different scenario's such as having the draw for the Vintage Air being off and now instead of a 30A draw you have 0A draw and now something else on that stud could draw up to 50A BUT if one were to still have the 50A draw on that terminal and then you switched ON the Vintage Air, then the total amperage draw would exceed the 50A rated fuse.
Also too, on the Vintage Air with them including a 30A breaker does not mean when it is on it is actually drawing that kind of amperage. It might spike when first turned on to 40A but then settles down to something like 20A even though you have the 30A breaker, the breaker is in place to protect the wire and the wire has to be large enough.
Since I do not know exactly what size breaker the Vintage Air uses, I just picked a random size of 30A. The breaker could be of another value so one just has to do the math once more things are known.