The Cummins ISB is a good fit for the Seneca but that depends on what you want to tow. It will climb western mountain grades towing a Jeep. My Tiffin DP had the same engine and the Seneca does much better on grades, i.e., I can feel the improved power to weight ratio (tranny doesn't downshift on rolling terrain). And, I'm getting better fuel mileage - averaging 9.5 mpg running at 65+ mph vs 8.9 mpg at 60 or less.
Re lean. The Seneca corners better than my DP with upgraded shocks. Speeds posted on curves were to be taken about 5 mph less with the DP.
2015 Jayco Seneca 36 FK
2016 Jeep Wrangler Unlimited tow
2010 Tiffin RED 36 QSA - gone
2011 Monaco Monarch class A - gone