If a player wants to use HAUL vehicles, they have to add a roadtype that supports HAUL.
I'm with Andy on this one - same as with metro trains, you need a metro type track to use them in game, or a parameter to make them run on electric rail. The grf can't change the active code for each vehicle based on what types it does or doesn't find. If one really wants to use RVs that someone has encoded as being HAUL type on regular ROAD, then could use a roadtype grf that sets ROAD as being compatible with HAUL.
Currently only one RV set uses HAUL - the NRT fork of Road Hog, which adds a few new very heavy trucks with a gameplay concept of 'increased capacity at the cost of incompatibility with the ROAD network'. If someone does decide to update HEQs, they will have to make that determination themselves. Up to this point I have generally been of the conviction that I neither want to add a HAUL type to my Road grfs, nor that I want to add HAUL vehicles to my RV grf - as I typically just build a tram or train for loads larger than a Road train. I might change this in the future.
eGRVTS would only have separation between non-electric tramways and electric tramways, otherwise maybe some trolleybuses but it doesn't have vehicles that would go on HAUL.
As you have stated, it doesn't need to use HAUL to benefit from a NRT re-write. HAUL is just one niche use case for NRT. I've long been most excited by the prospect of using steam trams without needing wires overhead.
Also, i dunno how to code an NRT grf,
Road Hog Fork, Trolleybi, and Mop Generic Vehicles are all GPL with NML code posted online. Start with one of those.
neither do i know how to have refits that give more articulations to the RV.
To be honest, I am not really sure this is worth is, as it can mess with station refits. As long as RVs are built as single units, It's probably best to just have 'truck' and 'road train' as separate vehicles.