Hi
First and foremost, when were the appliances last properly serviced?
A 1991 Van is now 16 years old, and largely because they are not used very much for most of the year many of these devices deteriorate simply by age, and possibly spiders or other insects etc. using them as nesting sites.
Simply by the need to ask your question I suspect you are not a qualified or competent gas fitter. As with all gas appliances they must only be installed serviced or removed by a competent person working to the gas regulations.
Best solution is to have the gas system serviced which should identify all the problems and leave you with a working and safety checked system.