Welcome to JOF!
Marker lights wired in series? I find that hard to believe. Just thinking logically about it, if you used 12V lights in in series for the marker lights of an RV, they would glow very dimly and also create a serious safety hazard if they all went out when one failed. Surely this person misinformed you. I've had marker lights on my TT (and TV) go out, but all the others remained lighted.
Unfortunately, I'm not familiar with the Seneca so I won't be able to answer your question. However, I'm confident that someone who owns one will be able to offer assistance.
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TV: 2009 GMC Sierra 2500HD | Crew Cab | Std. Box | 4WD | Duramax/Allison
RV: 2000 Jayco Eagle 266 | FBS | TT -- 1986 Coleman Laramie pop-up -- Still in the family!!!