What causes soft floors

I've got one with the sandwich floors and it is awful. I've got soft spots in all the high traffic areas. I had no clue about this when we bought the camper. I would avoid it at all costs.
I'm wanting to go to a 5th wheel, but really dread trying to sell this one. I've looked at repairing, but it looks like it will be a huge undertaking.

Does it matter which part of the sandwich is faulty. If it's the thickness of the plywood portion or the density of the foam either way the product loses its rigidity and creates soft spots.
Why the sandwich fails is less important than the simple fact RV manufactures are continuing to use questionable products and questionable construction methods. The consumer ends up holding the bag of soft floors!
I agree sandwich construction is generally found in "Lite" units.But I don't believe it's exclusive to light units.
The real bottom line is units that don't use sandwich construction are generally more costly which makes those units less desirable to $$$ focused consumers.

Can you point out the specific models of RVs that use T&G subfloors and conventional home floor structure?

I have never seen such setup in any RV I have been in so I highly doubt what you say is true..

Been in RVs from the 1960's and up and so far none of those were constructed like a conventional home floor with T&G.

The problems you state with sandwich floors is not 100% true in all cases. The problem stems from manufacturers reducing the thickness of the sub floor. I have a 1980's TT which has 1/2" thick plywood as a subfloor, but it is also glued (laminated) to the Styrofoam insulation below it and under the Stryofoam is as 1/4" plywood layer.

Modern RVs instead of using 1/2" plywood subfloor are using 1/4" or a bit thinner MDF or strand board to save weight and expense and that IS the real problem.

The only way you can avoid the thinnest subfloors is to make sure you are not choosing the "Lite" versions but then it now is heavier than what everyone wants to tow with can tow and not to mention far more expensive..

MDF and strandboard can no longer be avoided, the cost of real plywood will drive a lot of consumers away from the RVs..

I hear you Hunting Dog and I sort of agree with your upgrade what you can philosophy. Except sandwhich floors are sort of structurally compromised from the start. The foam will not remain ridged for the lifetime of the RV. It is inevitable that soft spots will develop in high traffic areas.
In some cases there is not a lot you can do in certain scenarios to repair this issue.
Sandwich floors are very common but they are really something to avoid.
In order to avoid the sandwhich floors you have to migrate to a more expensive unit. To complicate it even more, sandwhich floor are inherently structurally compromised. The floors become even more a liabilty when they are not supported properly or compromised by a duct are wiring chase.
Many are not aware that the sandwich floors are a liabilty, they don't understand why its important to have t&g sub flooring on their must have list.
To their credit sandwich floors are lightweight and provide insulation, but if the floors are not sturdy they quickly become a liabilty.

Beefing up the sandwich floors can be a simple task or it can be very involved.
Avoiding the floors from the start is the best solution. Convincing manufactures not to use sandwich floors is a better solution. But Huntingdog is probably right. SAndwhich floor are heere to stay for the foreseeable future

What causes soft spots in floors? What can be done to prevent that? How serious is it?

Soft spots maybe the result of moisture from a leak. They may also be the reult of poor construction techniques and inferior sub floor material.

They can easily be avoided however the manufactures have to committ to designing and building a better unit vs. building units with known construction flaws and selling them anyway.
On the other hand consumers cannot me so price focused that they refuse to buy the prperly built unit, because they don't wnt to pay a few grand more to get a unit that was built right from the start.

The soft spot we have is due to the heating duct and the way Keystone cuts/installs it into the floor. Its right where we step down off the steps from the bedroom/bathroom and into the living area. I've read where others have had the same issue, same spot.

jdc1 wrote:
Yep. Dryrot. Usually found under sinks, in front of shower/toilets, and sometimes a window or vent left open. Very seldom does it happen from below.

I would disagree. On some brands of newer trailers using foam sandwich floors that are only protected with darco. Rocks get thrown up and pierce this lightweight material. Driving in rain causes water to be thrown in the holes leading to break down of the wafer thin wood bonded to the foam. Mold moves in causing further damage.

Worse case scenario is a roof leak....water seeping down the walls and absorbed by floor.
When looking at used trailer's it would be prudent to avoid any and all trailer's that have soft floors. Imo

I noticed on mine there is a section right in the kitchen that is not as solid as the rest of the coach. Turns out that's right where the black water tank is...meaning there is not a cross beam between the frame rails for the width of the black tank...about 4 feet. So in that area, as far as I can see the 2x2 with 3/4" tongue and groove float happily over that span.