I bought these last year, installed them last summer, took one trip, winterized the van, and replaced them for a 50a combo unit when I added another battery. So they're basically like new. $110 for all three.
To install the DC2DC: You run a 4awg cable from the DC2DC up to the starter battery on the van. (possibly a smaller wire if the batteries are in the front of the van) And also connect a + & - cable to the house batteries (in my case, to my distribution panel. Watch the Renogy video on youtube to instructions.) Then you need to add a fuse (I can include the one had) and get a skinny wire to go from the DC2DC to the ignition box with a tiny auto fuse. My friend helped me with this, so you will need to get better instructions. I will take a photo of what that looks like installed. And I think I have a wire to include. Also, there's a little set on buttons you need to adjust to your battery type. I will take a photo of where they are, and I have the manuals to include.
Will this solve your problem? I don't know. 2x @ 100ah agms can't be drained below 50%, so you only have 100ah of battery for your power. If you drive a lot the dc2dc will absolutely recharge them, and I believe it even recharges with just the engine running, although that's not good for the van. An MPPT charger will absolutely be better than the one you have. So you are definitely upgrading. If your load is way bigger than the power you have available, and you don't drive a lot or get much sun, these upgrades won't help.
But the devices definitely a good deal. Nearly new and 1/3 the cost of what I paid.