If the house was built in 2001, it must have a cleanout.(required by code). Will also require an access for.pumping the solids tank.

Cleanout *may* be in/under house, which doesn't help.

Think your best bet is to install a T yourself (actually a street elbow, IIRC). It's just PVC, easy to do, BUT...I'm not sure there's a safe/proper way to do this. It will require preventing flow from house going into RV system, and ensuring a proper angle on drain line from RV (1/8" per foot)