Chile/Argentina/Uruguay

Some of my route (over 10 years ago) through Uruguay, Argentina and Chile was similar to yours albeit in the opposite direction - I can only remember a couple of nights when I was unable to find digs in populated areas and rolled out the bivvy as a plan B (although there were many times I camped by choice - especially in the riverine towns of Uruguay which almost without exception have campgrounds or yacht clubs in lovely surroundings).

The biggest stretch you have I think would be Mendoza-San Luis, I did that in a day but it's around 255 km as I recall and I can't remember there being anywhere to stay (or much more than vineyards and one or two petrol stations) anywhere along that section. The Andes crossing is not difficult as 4000+ metre passes go. From Los Andes there are little settlements and a few ski resorts dotted along the road which will have some sort of accommodation and on the Argentinian side you have Las Cuevas, Puente del Inca, Los Pentitentes, Uspallata and probably one or two others I'm not thinking of before you hit Mendoza.

As an alternative detour have you considered heading north from San Luis through Villa Dolores and over the sierras near Córdoba? This is a lovely area well set up for tourism - on the western side you have numerous balnearios nestled in rocky canyons and pretty towns like Nono (which has a totally insane museum of everything) and Villa Cura Brochero (Mina Clavero is the touristic centre of this zone and may seem a bit crass but has beautiful surroundings, and some great food). Then crossing the sierras (there's even a couple of secret routes closed off to traffic you can take) there are towns like Alta Gracia and the German settlement of Villa General Belgrano. I'm not sure if you are restricted for time but it might be nice to take a few slow days through that region to add some variety to what will be a whole lot of flat through the middle of Argentina.

Entering BA carries risk in terms of traffic and becoming a target for robbery but it can be done - everyone has their own idea of what constitutes safe so I won't comment on that but I have done it myself without problems and would have no qualms about doing it again. Get up early, be on the road by dawn and you'll avoid a lot of traffic and scammer/thief/murderer problems. Looking at your route I assume you would be entering from the north - which I think would be the best direction to enter the city from (you could even take a day off to explore Tigre and the delta on the way).

If you have your heart set on travelling light you could do it without a tent but you'll just need to be prepared to knock on doors and probably hitch hike in several areas - people are friendly (outside of BA) and eager to help but I would personally just take a tent.