OH boy... where to begin. The increased stability that a tag gives is unbelievable and yes I had a 40' 8 airbag non tag DP previously... GET A TAG... save a step. I went 10 years on my rear tire and then put them on my 10 wheel dump truck. I did put new front tires on at 7 years.
If you count the extra moving parts in in a IFS you will understand why I am not a fan of IFS. My Monaco RR10S is rock solid and I often put in 12+ hour back to back days... non IFS. When the OTR tuck go to IFS instead of a solid axle I'll consider IFS. Guess which rigs use a solid front axle and 10 airbags... Big Monaco/Beaver/HR/Safari, Foretravel, Newell, and Prevost converstions.
Extra maintenance of AquaHot/hydronic heating is only true if you do the yearly PM. In 12 years I've done it three times... Why? Because most of the time it's powered by the engine or the electric heating element/s. Maybe there is a reason why all the units you are looking at HAVE IT. Once you've had it you will not go back... same with a tag axle.
2' difference in length makes getting in NP impossible... You have got to be kidding. OK in Yellowstone my 42 becomes a 40'. We do three months out West without reservations staying usually 5 days/week (Sunday to Friday) in USFS CG because if you show up Friday there will be a Class B in a site that you would have fit in. We travel and resupply on the weekends. The exception is long holiday weekends where we usually stay put for 7-8 days.