Depending on what is said in your contract and receipt you could lose your downpayment (or a portion of it) if you decide to backout of the deal if it mentions the downpayment is nonrefundable.
Your deposit question would be better answered by the dealer you put the money down at..
As far as the sagging ceiling goes, some sag can be considered normal between the ceiling /roof joists as there is unsupported space between the joists. Excessive sag with discoloration of the ceiling panels would be a big red flag of a major leak in the past or ongoing leak in the present.
Lets face it, you are looking at 8+ yr old used RVs, some things to fix or address are to be expected..
