If you put the T between the main on/off valve and the first regulator, you're on the high pressure side of the system, and any appliance you wish to use will usually require it's own pressure regulator. Many portable grills, like Weber Qs, come with them. If you put the T after the first regulator, you're on the lower pressure side of the system, and appliances with their own regulators may not function well, if at all.
I just added a T between the main valve and first regulator on my 2018 Navion, and can now add a "Stay Longer" external tank, if necessary, as well as use my portable grill with it's built in regulator.
Mike. Comments are anecdotal or personal opinions, and worth what you paid for them.
2018 (2017 Sprinter Cab Chassis) Navion24V + 2016 Wrangler JKU (sold @ ????)
2016 Sunstar 26HE, V10, 3V, 6 Speed (sold @ 4600 miles)
2002 Roadtrek C190P (sold @ 315,000kms)