Ensuring the water works on your tiny home water tight is quite helpful; here are a few aids:
these red and black plugs are great from oatey are great for anything drain related; they can easily hold 10 psi (plenty for your drainage abs)
A couple of pressure gauges and and some pipe fittings with a pex at one end and a air hose nipple at the other, with a valve in between, allows you to pressurize the lines as needed (80 psi in this case)