Lazy Daze Owners' Group

In addition to what Joan said, the deeper you discharge a battery on a regular basis, the shorter its lifespan will be. This chart from the Lifeline Battery Manual tells the story. At 30% depth of discharge (70% state of charge), you can expect about 2000 charge/discharge cycles before the battery is unusable. At 80% discharge, after only about 500 cycles, you'll be buying a replacement. (The chart is for Lifeline AGM batteries; I'd expect the less expensive Trojan or Interstate flooded-cell batteries to show a similar curve, but perhaps with fewer cycles overall.)

"Since I'm low on battery bank..."

There are really two factors here. You need enough storage capacity in your batteries, but equally important, you need to be able to replace the power you take from those batteries. Driving is one way to do that, but you'd have to drive every day or two to keep your batteries up. (Idling the engine while parked is a last resort; it's slow charging and not great for your engine or the environment.) Running your generator, if you have one, will work, but your neighbors won't thank you for it.

Have you thought about adding a couple of solar panels? That's clean, silent power that costs nothing after the initial investment. And with today's dropping prices, the initial investment doesn't have to be a big one. Even a 100 W "suitcase" system can be help keep your batteries topped off.

"...have voltage stabilizers..."

I'm not sure what voltage stabilizers you're talking about. Can you fill us in?

"and have been priced out of RV parks..."

If you're talking about commercial RV parks are too expensive, you might want to try Passport America. For $44 a year they offer 50% discounts at 1,800+ commercial campgrounds. You only have to spend a couple of nights a year at commercial campgrounds for that membership to pay for itself. And you might try state parks, which tend to be less expensive than commercial parks.

Of course, there's always dry-camping, my personal favorite lifestyle. Which brings us back to solar panels. For boondocking, they can't be beat.

"I think I'll see how the below 50% discharge works."

As others have said, I think you'll be buying replacement batteries sooner rather than later if you do. Why not consider the alternatives just mentioned?

I don't understand.  Under canopy, how is a portable solar kit more effective than the 4 panels on the roof?

"We are often under canopy, as in Yosemite, so capacity matters."

I feel as if I'm missing something. Capacity matters, but it's no use without the ability to replace the energy you use. When you're parked under a canopy, your roof-mounted panels aren't producing any power, so if I understand your description correctly, your generator is what you use to recharge the batteries. In that situation, increasing storage capacity from 300 Ah to 500 Ah just means you'll have to run your generator even longer to get the batteries charged up to 100%, no?

It sounds as if what you need to go with those bigger batteries is more solar input. I heartily endorse Greg and Mike's suggestion of putting some panels on the ground in a sunny spot near your rig, to get additional solar power. My friends and I do this all winter. Since you already have a good controller (assuming it can handle more than 400 W), all you need is the panels. $250 will get you a pair of 100 W Renogy panels. Add some cable (I use AWG 8 two conductor cable) and you're in business. You can cobble up some legs to tilt the panels for maximum efficiency, but I was lazy and just bought $9 plastic folding step stools from Walmart. They fold flat and can be used for other things when not propping up panels. :-)