portable solar.

Almost certainly the solar is connected via Sargent.

Infinitely better is as Paul says and I have, is a dual battery solar controller connected directly to the solar panel and then the 2 batteries via inline 10A fuses/

For most folks 80w will suffice.

When we had 100w we never had any issues

However, as your van is new you may have warranty issues....

Perhaps Bargee.

But not having anything Sargent, thankfully, but even when we had on the two Warwick, we had 100w solar, dual battery Pwm controller all connected directly to the panel and batteries.
A single lead acid 110Ah . We never had , nor still do, with a similar system, any issues at all re power. Being static is irrelevant, our batteries are full by lunchtime......
In the case of the OP, his set up is different and, as you say, marginal.
It wouldn't be if conn as above imho

doog1948 wrote:having spent 3 nights at Watlington without hook up, the battery on our nuevo shut down. because the gas tank valve is electric this stopped the fridge. the 80 watt panel on the roof doesn't seem enough even with the sun beating down. we had alarms ringing on site at 2am. not popular. 
would a portable panel of say 130watts be a viable proposition or not?
Doug.

If your Nuevo has the same Sargent solar regulator mine had which through the EC700 was supposed to smart charge whichever battery needed it, it won't support you off-grid for long even at this time of year.  I like many others replaced my Sargent regulator with a MPPT Votronic DuoDig regulator that charges the LB with a trickle charge going to the VB and most important of all it bypasses the EC700. There is much about this on the Forum.