If the whole septic system thing goes south, consider using a holding tank, say 500 gallons maybe even less. Arrange with the local guy who does septic system cleaning to pump your tank on a regular basis. Have a contract with him... I have heard of this working on a temporary basis. The county IF involved may consider this, with a contract to empty/clean on a regular time frame.
This Might be better than trying to dig leach lines, but may be more expensive.
If you do this all "bootleg" this still might be a safer way of getting what you want to do.
Another option may be contacting your local "portapotty" guy he may be able to set you up with some kind of lower profile above ground tank to be pumped out by the portapotty guy.
Last resort just get a portapotty and run shower sink water out away from your trailer, I know in some counties in TX they can run their washing machine discharge on the ground .
Thats about all I got for you