I imported an obj model of a truck made by someone else in a different 3D program but it is outrageously huge; it exceeds 2 GB and none of the vertices are connected together so I can't use the 'L' key in edit mode to select connected vertices. I also tried using decimate and limited dissolve but both of them made the object unrecognizable and weird. Is there any way to just get the exterior of the truck (without using decimate/limited dissolve) without remaking it entirely? The other person made the file using Catia so I'm not sure how the export settings for that program would work. I tried manually removing and combining the vertices in edit mode, but it almost always crashes my program since it lags a lot.
i had models like that too before i was able to create my own projects... here's how i made most of them usable... to connect the vertices, simply select the coplete object in edit mode. then hit OBJECT/VERTICES/REMOVE DOUBLE. to reduce the size (# of vertices), switch to OBJECT MODE, go to MODIFIERS and select DECIMATE. there choose for UNSUBDIVIDE if the model is created mainly in quads. change the ITERATIONS value until the model still looks acceptable and hit APPLY. if the model is created in tri's, choose for COLLAPSE and change the RATIO value (in OBJECT MODE) until the model still looks acceptable and hit APPLY. hope this helps
Solution 1 - Shrinkwrap
You might have some success with the Shrinkwrap-Modifier. Create a cube that completely surounds your high-poly object, apply it's scale, subdivide it a couple of times to give the modifier some vertices to work with, and then use the Shrinkwrap modifier on it with the high poly truck as the target. The modifier should wrap your cube around the outer surface of the truck. When you're happy with the result, apply the modifier.
Solution 2 - Manual Retopology
The Shrinkwrap modifier doesn't give you very much control over the result. So even though you're looking for an answer that doesn't require you to model the truck a second time, that might be the best solution.
Use either the result of the shrink-wrap or the original truck model to model a very low-poly version of the truck over the existing base. This process also works if your viewport is lagging due to the huge number of polygons involved - you can simply delete the parts of the truck you're currently not working on to reduce the strain on your hardware, and re-import the full truck once you're ready to work on a part you've deleted.
Make the low-poly model it slightly bigger than the underlying high-poly geometry (or you might want to use Snap to Faces to stay as close to the original surface as possible). Once you have that, you can bake the details of the high-poly model into normal maps for your low-poly model, which should give good results even when you've drastically reduced the number of vertices.
For mesh retopology, I found the LoopTools and F2 addons very valuable. They should come with Blender, you just need to activate them. F2 especially helps you fill in faces when you have surounding geometry that helps guide the fill in process; it works wonders on well-modelled quads and speeds up your retopology process. I'm doing retopology in two ways: Either "Top down", which means I'm surouding the target with very large blocks and then use the loop tools to progressively get closer to the high-poly mesh, or "bottom up", where I start modelling with a loop of single quads and then extrude to finally cover the whole model. Which way I go depends on the underlying geometry; if it's fairly simple, I found the top down approach was quicker.