That just does not work with a MHP's business model. If you do that the home becomes legally part of land in the same way an apartment building is. The DMV no longer taxes the home, it is then taxed by the county just as all the other buildings and land are. Once you have the homes classified as real property, you no longer have a mobile park, but a bunch or rental cottages. The option of selling off the POHs will be closed to you.
Try to educate the lender. His idea would be shooting you in your foot. He needs to see that the park's value is in the lots not the homes. Show him a plan to sell off the park owned homes, which frees up capital to bring in more homes, which makes the park more valuable. Attaching the homes to the land will make the park less attractive to future buyers and thus, lowers the market value of his collateral.