Step 1 - Go to Manage -> Mechanical Setting -> Pipe sizes and segments. Make a new Segment for your RVS pipes, with the correct sizes, inside and outside diameters.
Step 2 - Create a new Pipe Type. In the Type Properties go to Routing Preferences. Select the Segment/material you made above and max and min sizes. Now you can at least model pipes with the correct diameters.
Step 3 - Make /get Fittings. This is the hard part. If you cant find the fittings you want copy and modify existing ones. You should think about what you actually want to do with these pipe fittings. Do you need to show every tri-clamp for visualisations? Or do you just need to ensure there is sufficient clearance space for the fittings? It may be better (Much easier, faster with less detail) to just model a fat cylinder that reserves enough space for the clamps.
Step 3 - Set the prefered fittings in routing preferences.