my recommendation is get a bigger panel. Last year I had a similar problem of trying to keep my 220ah lifepo4 (powering a fridge) charged with a 240 watt panel. I couldnt do it in the summer while powering the fridge and running my swampcooler. At first I tried to upgrade my batterybank by adding another 312 ah of lithium to just run the fridge.
But now I had 2 batterybanks (total over 500ah of lithium) and the 240 watt panel couldnt keep both batteries charged especially in winter, by november they where both limping along at less then 10 percent for weeks at a time.
I was going to buy a generator to just try and top off the batteries every once in a while but I had no room for it, I finally decided to buy another larger panel 365 watts. 2 weeks after I bought the panel both battery banks were at 100 percent, this was in the december timeframe, where the sun was still low on the horizon.
I have one of the smaller astrovans and the panel easily fitted on the roof, on your extended van it should easily fit, you can probably fit one of the 400+ watt panels.
Getting another battery won't help, it will only be another battery that the 100 watt panel won't be able to charge especially in winter. My fridge set to 27f uses about 30 ah in the summer, during winter it uses about 20ah.
A 365 watt panel (laying flat on the roof) maxes out at about 15 amps in winter, in summer it maxes out at 27 amps. Thats enough to keep any battery topped off.
Without the 365 watt panel, I would have needed a generator or find somewhere to plug-in to charge my batteries. I don't drive enough time for the alternator to be an option. After a year of use, the 365 watt panel has kept both battery banks at 100 percent, I havent had any more low battery situations.
I bought my panel off craiglist for 250 dollars, you can sometimes find good deals. Right now I see 380 watt panels going for 230 dollars. The only drawback is then you need a bigger mppt controller, but those are in the 130 dollar range.