Forester behind a motorhome - signal issues...

If the Forester has the same bulb for turn and brake that third light has to be wired back to the brake switch. Otherwise it will activate when the signal is used. If they are separate bulbs for turn and brake then third light works only on the brake circuit. Wiring may need to be checked out.
If the motorhome and the toad have a different taillight setup there are adapters for that.