The van's charger, as would be typically fitted back in 1996 will arguably not be a "charger" as such, but a PSU set to give a maximum supply voltage for everything including the battery, not greater than 13.8 volts.
That supplied to a battery achieves a charge, and is as high as an uncontrolled supply can be whilst not unduly "gassing off" the battery. It is as rudimentary as that. It also protects sensitive electronics designed for use on nominally 12 volt DC systems.
The solar controller ought to be somewhat "smarter". If not fooled by the van's charger it could recognise if the battery state of charge is low enough to warrant a Bulk or Absorbtion charge and take it up to 14.4 volts. If it thinks the battery is charged, it will drop into a float charge of 13.6 volts. Note, somewhat below the van charger's 13.8, as 13.6 better protects the battery from gassing.
My thoughts are, however, that if you plug in the EHU, the solar controller will simply "cop out" of bothering, as then it can't determine if the battery is in a low state of charge to warrant going into even the absorption phase. Plus as its float stage is lower than the PSU's regulated voltage, it can't see it need to assist there either. The van's charger will carry on as it always had, and nothing is harmed.
If it is a roof mounted solar system always active, then the probability is the battery will be fully charged when you arrive on site and plug into the EHU.
From then on the van's PSU will cater for all demands within its power output, and the battery sit there reasonably content with its status, but on a slightly over generous float charge, until the EHU plug is again pulled.
Again, no worries, basically because the van's charger in this 1996 case will not be sophisticated enough to try any smart battery charging routines, here it's not that type of device.