Gyre, as 007 pointed out, more and more vehicles are limited by thier drivetrain. More US builders are going to ZF and Gerhard? boxes. These boxes are generally engineered with very little "reserve".
Brakes are getting better but drivetrains are getting weaker.
Todays engineering decisions are made by beancounters, not engineers.
1 fleet that I'm familiar with, Dryers ice cream bought new Sterling's. They won't stop. The brakes were speced out by beancounters and will only work OK until you warm them up good.
The Richmond trans might not be good for high weight. Transmissions produce a lot of heat. Many manual trans now come with a pump and trans coler. I think that the Richmond is more suited to drags rather than constant heavy loading.
If you want to pull the Airstream with the lowest investment and the highest safety, you should look at IH. You could buy a new F-550 but the cost will kill you.
It's easy to find an old IH with a DT-466 and an Allison. They're 185 to 205 hp. They go for about $ 2000. That way you'll have a heavy chassis with good brakes. The parts are cheap and you can change it to a 2 speed axle and you'll always have the "right" gear.
They aren't bad on fuel and the cab is moderately low,,,wind resistance.
I took motorhome caravans to Mexico and Alaska for 10 years. Believe me, good brakes are real important when pulling a trailer. Also, a heavy stable chassis keeps you in control much better.
A ball type hitch tends to "wag the tail" If the tail weighs more than the dog,,, it's the dog who gets wagged.
You can buy an old semi-tractor, but the cabs are such high profile, you can't expect good mileage. Freightliner is the best.
As far as the cooling fluid, there are lots of good additives and bases. Now Cool is highly recommended too.
The other thing that's important to the cooling system is electrolysis. The new trucks have so much electronic'electrical stuff on the engine that electrolysis is becoming a huge problem.
The best solution is a water filter. Most mechanics don't realise that the water filter contains a "sacrificial anode" This anode is disolved by ion exchange. This prevents your radiator from being disolved. Dis-similar metals always have an ion exchange.
That's why boats have zinc anodes attached to the metal parts.
Mechanics don't change the filter because it isn't dirty. They rarely know about the anode.
There is a company in Texas that builds the DT-466E as a propane engine in case you're interested. It's 250 HP
I'm rambling, gotta fly.
I don't post things because I believe that they are the absolute truth. I post them because I believe that they should be considered.