I concur a chassis review for warn parts, and revalidated settings and alignment should get your Manga riding nicer. I believe the Intrigue chassis on up to Affinity (key word here is 'believe') have one at least four more air bags then our 2004 Allure model. Two bags each axle side of the Front and Drive axles. Don't think the Tag's have more then a single bag, like the Allure. So, your ride should be a bit more smooth then the Allure.
I'm also not sure what shocks came stock in 05 on the Magna. But the 04 Allure, in the 40' Tag model, had first generation of Koni Adjustable (SP1). The Rear and Tax axle seem supported by these. The Front IFS combined with the weight, were just not up to the job. Even with our shocks set on firmest 5 position, I felt the coach front end was very easily 'upset' in small dips, or twisty roads. We ended up going with Road Kings all around, and I also added Super Steers Motion Control Unit (MCU's) to the front and rear axle. These help the air bags 'set' faster, via a sort of air 'choke' valve. I'm very pleased with the handling of the coach. However, Road Kings are firmer shocks then you have now, so factor that if considering them. (I take handling for emergency situations over softer riding chassis. Just my preference.)
Road Kings are expensive. During my research, and after talking with Koni, I found out they were then (3 years or so ago now) coming out with a third generation of the Koni Adjustables, the SP3's. These were much more robust then the SP1 generation. I suspect that most owners would be very pleased with the performance of the Koni SP3 shocks. (And heck, many felt the 2004 Allure coaches road great on the shocks that I felt were just too small to handle the load. So to each their own.)
One bit of hmmmmmmm info, is that our little T28 Bounder, on the F53 18K lb chassis, used the same exact shock model number as our 40' 35-37K lb Allure...
Now all that info about our 2004 Allure, could transfer over to your 2005 Magna - especially if the Koni SP1 Adjustable shocks came stock. Worth checking and asking.
On front seats. I find ours to be comfortable most of the time. The 1st owner only had 13K miles on the rig, so not too much usage. I do find I have to adjust the seat back up pretty straight, and sometimes adjust it back and forth a notch as I drive, to allow the back to be in a different position. Lumbar support options are on the low side, so possibly a cushion is in order as it 'fits' your back.
Best of ongoing luck with your rig, getting it dialed in will pay dividends as you put some miles on it