The modular tanks (except for the modular lander tank) appear to have negative masses in the VAB and thus float in midair when I launch them. I am aware that this issue was solved when RealFuels is installed, however with the rest of RO installed the bug appears again. I am guessing it has something to do with RO rescaling and re-weighing parts?

Running KSP ver 1.7.3 and the latest RO and SSTU, no other mods

I am aware that this issue was solved when RealFuels is installed, however with the rest of RO installed the bug appears again.

If it works with 'stock' RealFuels, it should work with RO. All that is needed is a simple patch to the SSTU fuel tanks configs ( https://github.com/shadowmage45/SSTULabs/blob/master/GameData/SSTU/ModIntegration/RealFuels/RF.cfg ) Either way, I am not part of the RO team, and cannot directly provide fixes for their issues. I have provided the functionality for them to use, and the responsibility to make use of it is theirs.

To confirm -- you say that the tanks work fine with just 'stock' RealFuels installed, but then revert to having issues when the full RO pack is installed?

In order to investigate further to confirm where the problem is, I would need to see an excerpt of the SSTU MFT fuel tanks' config out of your ModuleManager.ConfigCache (please not the whole file, I just need the config for one of the SSTU fuel tanks), to see what the final config for those parts looks like. 95% likely that the RO patches are not setting up the interop flags properly for those, which then manifests in the issue you are seeing. (also possible there is some other bug or conflict, but this seems to be a simple patch omission)

Heres what (I think this is the right one) says about one of the tanks in the ModuleManager.Config.Cache

UrlConfig
{
parentUrl = SSTU/Data/ModelData/ModelData-MFT-A-Tanks.cfg
SSTU_MODEL
{
name = MFT-A-0-5
modelName = SSTU/Assets/SC-TANK-A-0-5
orientation = CENTRAL
height = 2.5
diameter = 5
volume = 49
cost = 0
upperDiameter = 5
lowerDiameter = 5
upperProfile = cylinder
upperProfile = uncapped
compatibleUpperProfile = cylinder
compatibleUpperProfile = uncapped
compatibleUpperProfile = capped
compatibleUpperProfile = inset
lowerProfile = cylinder
lowerProfile = uncapped
compatibleLowerProfile = cylinder
compatibleLowerProfile = uncapped
compatibleLowerProfile = capped
compatibleLowerProfile = inset
minVerticalScale = 0.5
maxVerticalScale = 1.5
textureSet = SSTU-MFT-A-Stripes1-Stringers
textureSet = SSTU-MFT-A-Stripes2-Stringers
textureSet = SSTU-MFT-A-Stripes3-Stringers
textureSet = SSTU-MFT-A-Stripes4-Stringers
textureSet = SSTU-MFT-A-Stripes5-Stringers
textureSet = SSTU-MFT-A-Stripes6-Stringers
textureSet = SSTU-MFT-A-Stripes1-Plain
textureSet = SSTU-MFT-A-Stripes2-Plain
textureSet = SSTU-MFT-A-Stripes3-Plain
textureSet = SSTU-MFT-A-Stripes6-Plain
textureSet = SSTU-MFT-A-Soyuz
textureSet = SSTU-MFT-A-Orange-Plain
textureSet = SSTU-MFT-A-Orange-Stringers
textureSet = SSTU-MFT-A-Gold
textureSet = SSTU-MFT-A-Silver
}
}

Thanks... close, but not quite the config I was looking for :) (I should have given precise names... sorry) What you posted was the config for the model definition for one of the tanks, rather than for the actual Part that uses the models.

Looking for the config for the SSTU-MFT-A part. Try searching for 'SSTU-TANK-MFT-A' in that same file, and it should locate the specific config in question.

First result for SSTU-Tank-MFT-A

UrlConfig
{
parentUrl = SSTU/Parts/Tanks/SSTU-TANK-MFT-A.cfg
PART
{
module = Part
name = SSTU-A-Tank-SM-IV
author = Shadowmage
TechRequired = basicRocketry
entryCost = 16000
cost = 6148
category = FuelTank
subcategory = 0
title = Modular Service Module, IV
manufacturer = SSTU
description = Level IV Service Module Tank similar to ones used by Apollo. These are used to store life support, payloads and electric charge. These are heavy tanks with poor utilization that simulates the supporting structures and systems needed for the specialized payloads. <color=green>Max Utilization: 75% <color=white>Can Hold Satellite Payload, Sounding Rocket Payload and Life Support Supplies (as well as regular fuel)
tags = tank, fuel, ?lfo, ?lf, lh2, hydrolox, kerolox, cryo, resource, storage sstu
rescaleFactor = 1
node_stack_top = 0,1,0,0,1,0,2
node_stack_bottom = 0,-1,0,0,-1,0,2
node_stack_noseinterstage = 0,1,0,0,1,0,2
node_stack_mountinterstage = 0,-1,0,0,-1,0,2
node_attach = 2.5, 0, 0, 1, 0, 0
attachRules = 1,1,1,1,0
mass = 1
crashTolerance = 14
fuelCrossFeed = True
breakingForce = 2000
breakingTorque = 2000
maxTemp = 773.15
gTolerance = 100
maxPressure = 20000
skinMaxTemp = 873.15
MODEL
{
model = SSTU/Assets/EmptyProxyModel
}

Was there anything below the MODEL node?

There should be a MODULE with 'name = SSTUModularPart'. And inside that there should be a value 'subtractMass = false'.

Edit: Pretty much that is what I'm looking for -- to confirm the presence or absence of that line.

Thanks.

That points to whatever patches RO is using being incomplete / incorrect. The solution will have to come from RO, by implementing the patches properly.

Notably here: https://github.com/KSP-RO/RealismOverhaul/blob/master/GameData/RealismOverhaul/RO_SuggestedMods/SSTU/SSTU_Tanks.cfg#L32-L39

They need to add the following line:
%subtractMass = false

I'm personally not sure why SSTU's RF patches aren't working in such a setup (as I'm already patching that line when RF is found), unless RO-RF has a different modname than stand-alone RF.

Thank you so much for the help, i just tried this and now there are no more negative mass tanks! Also turns out that RO has problems with displaying mass correctly for any fuel tank - not just sstu tanks. Luckily none of them have negative mass

under the config for SSTU-TANK-MFT-A 'subtractmass' is not listed at all