As an owner of a of a Hymer Grand Canyon S, I can speak with some authority and hopefully can put your mind at rest.
Firstly, lets be honest here, going on any FB group including this one and you will read of problems, upgrades and modifications.
You are correct in saying GCS lacks insulation, however this has been modified on the new model year. It is however relatively easy to insulate areas of concern if required but to be honest I've never found it a problem as I don't camp in minus -10 °
The main area of concern is the area above the cab which can cause water droplets to short the control panel. Remove headliner, add 19mm of insulation and refit, again rear door panels remove easily and its a simple job to insulate if you really need it.
As mentioned its not been a problem for me and to be frank its about as well insulated as a California.
As for mold this is normally caused by lack of ventilation in the poptop void over winter and the storing of the aluminium ladder.
Unlike the Cali when the roof is closed all bedding and ladder can be left inplace, if the ladder is left in cold temperatures condensation forms on the ladder and mould develops. Regular venting stops this issue.
I to looked at the Frankia at the Nec, in my view the build quality of furniture is poor as previously mentioned the placement of stickers to cover cabinet screws is unacceptable when spending 135k, I'd seen enough after 5 minutes it just looked poor quality-see below.
3 1/2 years with the Hymer my only problem has been a faulty micro switch on the kitchen tap and one broken cupboard hinge (operator error).
More than happy with the brand, I believe its good value, in 2020 with no discount the van cost 79k fully loaded, around 110k today.
I've since spent around 15k in modifications plus next month plan to have upgraded front and rear shockers fitted in Germany.
My biggest disappointment has been the Mercedes dealership that maintains the van and the high cost of servicing.
The supplying dealer Campersales Burgess Hill were excellent.
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