Anything for a motor home will probably be four prices at a dealer. I might be tempted to install a Crane adjustable vacuum can and vacuum timing limiter plate....The 99600 is adjustable for the RATE at which vacuum is applied to the advance function....
http://www.summitracing.com/parts/CRN-99600-1/
The 99619 is adjustable for the AMOUNT of vacuum applied to the advance function....
http://www.summitracing.com/parts/CRN-99619-1/