Over the years I have learnt that skimping in the wrong areas is both counter-productive and costs you more in the long term. Some tools may seem a luxury but can pay you back easily with what they give you. Other tools are real one-offs and maybe were not a worthwhile buy.
So the first post says to me that the maxcut software is a waste of money (I think it is free?) and so is a bench guide (to make costs staples, wood glue and a couple of ply sheet off cuts).
I say don't knock it unless you try it. On the Saw Guide, I am sure you have a straight edge that provide you with being able to cut straight lines. We have all done it that way, but the reason professional tool companies and professional tradesman use some tools is that is saves them time and money despite having to pay up front for the tools.
I would guarantee that I could cut, using my homemade tracksaw costing about £3 in materials, better and strighter pieces of ply faster than you could with with your straight edge (and just to say I am no carpenter by any means. I just found they make that much difference!)
To look at the general question - what tools do you really need to make a campervan? you could look at it this way.. If all your tools were stolen, or blew up in a explosion, or whatever, so you had to start totally from Scratch and you wanted to rebuild your Camper, what would you buy given you were on a sensible budget?
This would be mine (basic things like Stanley-type knife, sandpaper, etc are assumed available or bought)
Circular Saw. I think I would be inclined to just get one of the Mini Circular Saws rather then the 'standard' ones as they provide more flexibility.
Track Guide. I would make a track guide to go with the saw. For the amount of time saved and hassles removed with a guide, it is a no-brainer IMO
Jigsaw. Essential Tool, and could in many ways replace a circular saw if on a very strict budget.
Square. Always useful to have
Hole Saws. These I regard as a near-essential. It is possible to work without them using original drill bits and a jigsaw and sandpaper, but the amount of time and effort and improved quality of work from a Holesaw makes it totally worth getting.
Hand Saw. I can't remember the last time I used one TBH. Not in the last year or 2 anyway, but they are inexpensive and worth having one around.
Kreg Pocket Hole Jig. I am a fairly recent convert to Pocket Holes for woodworking and now I wouldn't be without them! A Kreg jig is not very cheap, but IMO worth the money and would be on my list.
Router. Got them (standard Plunge Router and a Trim Router) and they are certainly useful and would want available, but would be a secondary purchase if on a strict budget.
Work Table. I tend to just use a standard table with clamps, and a sacrificial board for the circular saw cuts has always suited me. Maybe buy a couple of A-Frame legs if no table around to commandeer?
Cordless Drill. An essential item. For the type of work involved in a self-build, a Cordess is much more useful than a Corded Drill and will almost certainly have enough power and life.
Cordless Dremel. May sound a luxury, but I found my Dremel invaluable. And I do mean DREMEL. there are lots of similar Dremel-like tools available for less and I have bought a couple but there is no comparision to the real McCoy.
Wire Strippers and Crimpers. For general wiring, standard Wire Strippers and Crimpers are fine. If doing a lot of wiring, it is worth upgrading to better tools. i.e. I use a Ratchet Crimper. I changed to this from a basic Crimper after I was getting blisters (literally) due to the volume of crimping I was doing.
Multimeter. An essential if doing ANY wiring. end of.
Pliers and Cutters. Another essential in the toolkit. Cutters sized to suit the job. I have and would rebuy both fine cutters for small cable and heavy duty cutters for the thicker cable (the ones I have are great for anything I hae tried - so far upto 70mm2 cable. Use same cutter for plastic pipe
Screwdriver Set. Selection of Flat Blade and Pozidrive (more common than Phillips) Screwdrivers. If used for Electrical work as well, preferable to get VDE type screwdrivers as added precaution.
Selection of spanners and an Adjustable Spanner.
I am sure there would be other tools as well, but the above would be in my shopping basket for sure to have before I started a build.
Another tool I have which might seem a gimmick and would not be an essential BUT I have found very usefulon occasions is a Mini Chainsaw!
This has allowed me to make adaptions to furniture that is already installed and where other tools cannot be used.