The floors also were very uneven - indicative of some joist/support issues underneath.

Gravel is usually the bet material but you also want to make sure your soil is solid underneath otherwise the weight of the vehicle can cause it to sag over time and might create other irrigation issues.

Your best bet without taking up the finished floor, is to just put solid blocking underneath.

Conveniently, it seems most conversations about umbrella policies don't mention GCL's underneath them.

downspouts pouring out water next to foundation, negative slope of outside soil or concrete, clogged gutters etc...

Ask your realtor to check if expansive soils are common in the area. 

Need to make sure they don't cover with rock, until we "slip" more grid material underneath at the end and extend the spread area.Trucks will drive straight over and spread behind them. 

@Christopher J Lemmon In my area, where the soil tends to be sandy and will drain well, we budget $35,000 and hope for $20,000 - $25,000.It's hindsight now, but when writing an offer, I always include a contingency that lets the buyer either get a price reduction or cancel the sale based on the outcome of the septic inspection. 

It's not in a mobile home park and he owns the land underneath the mobile home. 

Is it known to be particularly soft soil in the area?