That fluid didn't look terrible at all. Lubegard will definitely not hurt anything and it's cheap enough. The dry fill capacity of a C6 is about 12 to 13 quarts so a full bottle is fine.
Yeah, I was shocked it wasn't too bad looking. It may have been sitting in there for 15-20 years though since it got a lot of upgrades around then RV wise (new fridge, water heater) but then it was severely neglected for the next 15 years and allowed to leak a lot. It's in rough shape RV wise but it's still cheaper to fix wood rot as needed than to buy a new RV. Used RV prices are through the roof right now to the point of it being stupid.
The engine has developed an oil leak that I suspect is the rear main seal but it's not horrific yet. I haven't put any additives in the engine oil yet since it's manageable. I have been using TSC's Traveller oil since it's about the cheapest 15W40 oil you can get when it's on sale.
I'll have the shop do that rear main seal for me whenever the transmission gets pulled out for a rebuild. It doesn't burn as much as oil as much high mileage F250 and the steering gearbox isn't worn out (like they tend to be) so I suspect it's truly under 100k miles.
The differential oil was disgusting and was probably original. I put a reusable LubeLocker gasket when I changed the oil 2 years ago. I have since changed it a second time last year when I was doing rear brake work. I use cheap SuperTech conventional gear oil since it's an open differential, no LS additives needed so it's cheap and easy to change with that reusable gasket. No cleaning off RTV/paper gasket every time and the oil is about $15/gal. The Dana 70 takes just under 1 gallon to fill so it's perfect.
My memory is poor in regards to how much the C6 took to refill. It's odd that the info I was seeing online the other day when I looked up the capacity was saying 7 (5 in pan + 2 in TC) which did seem low. But now some results I'm finding are saying 11-13. Jegs claims 7qts in a stock (RWD?) pan and 2qts in TC:
https://www.jegs.com/jegs/jegsautotranscapacity.html It's one of those things where there's tons of bad info out there, I guess.
I went under and did about a 3/4 turn out on the vacuum modulator adjustment screw just to see if it might somehow lower the shift points but changing that part out a couple years ago and adjusting it a few times didn't seem to do a darn thing.
I wonder if the vacuum line to the vacuum modulator is somehow clogged up or a steel line crushed somewhere. I replaced the short hose that connects the vacuum modulator to the steel line down by the trans. I can't recall if I checked for vacuum down there or not. I guess I can look into that since I have a HFT vacuum/fuel pressure test gauge that I carry with me.