Grey Water Drain Venting

[Blueoval78]

Hello everyone, I just purchased a park model trailer with no holding tanks that I am using as a cottage on a remote property. The plumbing drainage that came in the trailer is this: 1.5" drain pipes coming from the kitchen sink (long run under the trailer to the bathroom at the back of the trailer), bathroom sink and shower, all tying into a 3" drain pipe / vent stack that goes through the trailer roof. The 3" drain is simply open under the trailer at the moment since it is intended to be connected into a park sewage system. My trailer is parked on rock, so I cannot make a septic system and have opted to use an above ground black water holding tank and simply drain the grey water on the ground. To achieve this, I have to cut the 1.5" grey drain lines, cap the stubs going into the 3" and combine together into a drain pipe running out behind the trailer. My questions are:
1. Should I include venting in this new grey water drain line?
2. Would it be sufficient to stay with 1.5" for the 3 lines to tee into or should I increase that size?
3. If I do need to vent, I don't want to start tearing into walls and roof to vent through the trailer so how high would I need to go to vent properly? If it needs to be high I guess I would have to come out the back of the trailer and have a pipe run up the siding which would not be pretty but functional

Thanks in advance for any help or advice!
Chad