Kristy - Lots are expensive and so are septic systems. How large is the parcel the mobile is on? Can you subdivide it and be able to both, build your house and have room for the mobile? Subdivision and septic requirements vary by county. In Williamson County you need a minimum lot size of 1.0 acre to have septic. Just north in Bell County you can do the same thing with only a 0.5 acre lot.
I'd either lease or owner finance the mobile all day long. Think about it - it's a cash cow that will provide long term returns if you structure the deal properly. It can be a challenge in this area to realize a 1% rent/cost ratio on a rental if it isn't in a war zone. If the mobile is worth even more than offered, let's say $20K, the ratio would mean rent would be only $200/month. I'm sure you would be able to get $800-$1,000/month for the double wide in rent. If you do an owner financed sale, it could get even better.
If you can't subdivide your lot and are 100% set on building your house there find an inexpensive lot in a rural community and move it there. This is far more valuable long term. Don't let it go.