You can avoid using a plastic base pan entirely by installing a Dometic auxiliary drain kit, part # 3107688.016.
These kits are used on just about every Duotherm installation that requires the use of an internal drain hose like Airstreams and most other quality RVs have.
The kit consists of 2 catch cups that are attached directly beneath the drains. These cups have molded-in nipples which connect to the supplied tubing. The water drains from the tubing via a 'T' fitting that connects to the interior drain hose. An extra gasket is also included to raise the unit slightly to provide clearance for the drain cups.
You might check it out!
They DO work well!!