Hi! Just tried out your truck. Some things I noticed/thought about during testing:

• Cars have a fuel status bar next to the fuel slots. With 8 fuel slots, there is hardly any space left horizontally in the GUI to display the fuel staturs bar; therefore, it becomes so short it's almost useless. Couldn't you put that bar beneath the fuel slots, or would there be some way to extend GUI width to more than the width of 10 inventory slots in the trunk -- or are these GUIs unchangeable?

• If you left-click a car, there's an icon of it (showing, as I just realized, also direction and -- if damaged -- health of the car). In the case of the Heavy Truck, the image doesn't fit into the available space, so I guess you should scale the icon a bit. The truck from this mod has a bigger collision box than yours, but is displayed completely, while yours is cut off at the edges.

• The heavy truck looks like a dump truck: ideal for bulk material cargo (ores, coal, sand etc.), but not suitable for stuff that comes packed in crates or barrels. I haven't checked yet, but would it be possible to apply inventory filters so that the heavy truck could only be used with bulk materials? ADDED: Limiting the types of cargo would give an incentive to players to use a wider variety of vehicles, so it would allow not only for more realism, but also make the game more interesting.
  (Thinking some more about this, setting fixed inventory filters probably would only make sense if any of the other mods was active. But if it was possible at all, checking for the presence of the other mods should be trivial.)

• Balancing: You've reduced the inventory size of vanilla cars/tanks, but there are other mods that add vehicles. The truck from LJD vehicles and this crawler both have less weight than your heavy truck, but have 160 instead of 150 inventory slots. In this light, wouldn't it make sense if you increased the inventory of the heavy truck (perhaps 250 to 300 slots)? I think it would also be good if it went much slower -- would increasing the friction accomplish this?

I read and agree with a lot of the things you've pointed out, but this mod was made "for fun" mostly, I didn't put that much effort into it at first, but seeing how much people seem to enjoy it, I'm considering overhauling some of it.
As far as other mods go, I'm not super sure how to restrict THEIR inventory as well, without adding every vehicle known to man in the mod files, but as far as a mega ramp up in this truck's inventory goes, it'd only be fair to add another major downside to it, don't you think? And that might come in the form of a much harder recipe, or yet another tax on fuel consumption and pollution, and this truck is supposed to serve as a supplement to AAI when that hauler just isn't cutting the mustard, and you don't mind using inserters, or as a quick and dirty setup when you don't have the resources for a train, or you don't want to set up a train for one patch of like, 100k ores you could crate up and truck back.
This kind of scratches a niche itch I had with a friend of mine when we were playing, setting up outposts and forgetting them (because we're not smart), and realizing we had a thousand crates of coal just chilling in the wasteland and wanted them back quickly. Three trips walking, a long railway we'd only ever use once, or a big tonka truck? It felt right at the moment.
As previously stated, I'm mulling over some ways to rebalance and basically make this a "real" mod and not just a toy as I play through a new bob/angels 18 go around, and see what realistically feels right and what would be useful as more than a decoration. I do appreciate your input though, and I'll take a lot of it into consideration!

forgot to mention I have no idea how to remap the GUI and the fuel bar never served me anyway. That might be my next step in learning how to be a real ModMaker(tm)

I read and agree with a lot of the things you've pointed out, but this mod was made "for fun" mostly, I didn't put that much effort into it at first, but seeing how much people seem to enjoy it, I'm considering overhauling some of it.

Nice. :-)

As far as other mods go, I'm not super sure how to restrict THEIR inventory as well, without adding every vehicle known to man in the mod files,

But you know what cars are in a game -- at least by the time data-updates.lua is run. You've just to check for data.raw["car"], that's the type car-entities will have. Then you could check data.raw.["car"][other_car].weight and change data.raw.["car"][other_car].inventory_size in proportion to your settings. Of course, loading order of mods is something you can't control without knowing all mods that provide new cars. So it may be that some other mod overwrites your changes again.

(Ooops, just checked: the crawler has a weight of 20.000 vs. 10.000 of the heavy truck, so my suggestion wouldn't work really work for it. However, the vanilla car has a weight of 700 and 80 slots, this truck weighs 5000 and has 160 slots, so it would work at least for some vehicles.)

As previously stated, I'm mulling over some ways to rebalance and basically make this a "real" mod and not just a toy as I play through a new bob/angels 18 go around, and see what realistically feels right and what would be useful as more than a decoration. I do appreciate your input though, and I'll take a lot of it into consideration!

How about increasing the weight of the heavy truck even more? But I'm a bit disconnected at the moment: too much modding lately, too little time for real playing, so perhaps other people may be better judges in regard to balancing. :)

Rebalancing the truck by increasing build cost, fuel cost and inventory size was a sufficient fix in my opinion. I edited the mod for my own use by only increasing those.