New a/c installed flush against my Mobil home. My a/c tech says it should be 8" away from?

The installer should have left the installation manual with you. If he did not, call & ask for it. Placement of the equipment will be in the manual.
If I'm not mistaken, federal law states any manuals or other paperwork which came with the equipment must be left with the homeowner.

If it is placed against the house, your efficiency drops since you lose air flow to roughly 25% of your condenser coils.