How to value mobils that are already financed by the park owner?

I am looking at a small park that has 20 lots each having a  unit on it. Each  unit pays 230 rent and the owner has sold the units to the renters on a contract for about 15k on a 15 year note.      The mobils are  70"s-80"s., single wides.
Owner collects rents and mortgage payment each month.
Owner wants to sell the park including the notes and units.     I am wondering how to value the units? I feel they are not worth the price that was paid for them and the time frame for the contract seems excessive
considering their age. Owners do not have very much down in them …   Should I value the units at what I think they might be worth ( 5000) each or is there another way I should look at them?

Central Illinois I could find a half a dozen 1980s single wides in decent to good living condition (could rent them out as is to varying quality of tenants) for about $2000. Someone down the road from my park had 70s units for sale and I’m not sure that she didn’t end up giving them away. Started at $2k and I last saw them advertised for $500. 

They 15-year term is way too long.  Tenants tend to leave after 7 years if they’ve not gotten title to their home.  The key variable is the condition of the homes inside.  I’d say $5k/home would be somewhere between ‘fair’ and ‘the maximum they’d be worth.’  Run a test ad, see what your market will bear for those homes sold with at least $1,000 down.  If you don’t see eye-to-eye with your seller, offer to let him keep the ‘valuable’ home notes and you’ll just buy the land.  That’ll probably get him to see your valuation is more in-line with reality.Best,-jl-