I've only got about 200 miles of driving time on my rear-engine Thomas so I haven't totally gotten the feel for it yet. It has air throttle and it does have the lag you're talking about but I'm not quite sure it's actual lag; kind of like you're talking about with the air brakes. I think it's that the throttle mechanism back there in the engine compartment is moving right away but it has a certain amount that it moves before there's much effect. The throttle pressure is so light (and so consistent) it seems you have to "think" change rather than "feel" change. For instance, in my front-eninge cable throttle International Blue Bird you know when you almost have the thottle to the floor 'cause you're hoping someone else comes along to help hold it down! On the Thomas it's as easy to hold it wide open as it is 1/2 throttle. So, really now that I think about it, it seems like a "feedback" thing...like with air brakes you have to adapt to a different kind of feedback than with mechanical linkage.
I came this
close to buying a transit bus just before the Thomas. I had my sights on an RTS; they were orginally built by GM then sold to TMC (the folks that build the MCI coach), and then a couple more owners. They're still building new ones after 30-odd year.
To my eye they make the nicest looking conversions on the road; not as tall as a road coach, sleeker than a school bus, rear-engine, 102" wide (most), 40' long (most), all stainless frame (it's built in 5-foot modules) so it's strong like a school bus, air ride suspension, and more. They're also a lot more complicated than a school bus and don't have the road clearance (same situation with the road bus) the skoolie does (if it matters to the converter), and the coversion process always seems to be a lot more involved. For one thing it has curved sides; looks good but makes building the interior more of a challenge.
The thing that plagues them, and seemingly most of the road bus conversion I read about too, is RUST! It makes it tough to just buy one on eBay or such if you can't actually go and inspect it (thoroughly!). They also usually have a lot more miles on them when they're sold (typically over 500,000) and that makes doing your homework on the engine and drive train a lot more important.
Still...if I had had a better feeling for the seller of the RTS I was looking at on eBay I would have bought it. I just never got that warm fuzzy feeling and I got nervous. The particular bus I was looking at also had a 5-speed ZF automatic; in researching that tranny I found most folks thought it was a superb transmission...unless it broke. The quip I remember was if it's broke...so are you! [Although I never verified the information a $15K figure was talked about for a complete overhaul!]
Those Gillig's sound nice (Gillig's are popular out west here and I'm a fan anyway)! If you converted one I'd still talk to you
. It's only those uppity mega-coach converters I steer clear of
! [It's a joke son! If I had the bucks I might go that way too...but only if I learned to weld.]