Does it actually work and increase profits without turning off potential guest? 

If you're suggesting he pull equity out of current residence, use it as a downpayment on new residence, and turn current residence into a long-term rental, that's a valid strategy, however he would have to evaluate ROI on the potential rental, and be able to qualify for the loan on new residence with a higher DTI ratio.

Go to REI meetup groups, put up some bandit signs, do a direct mail campaign, start calling people, go door knocking, just do something to get yourself out of your comfort zone and start working.

double wide on land In our area will be worth 200k or more itself.. now maybe in a park.. but then you have restriction in most parks here that you cant turn your MH into a rental.. it has to be owner occ. 

Turns out it was a "handshake agreement" and the other parties weren't interested in making it legal.

And I can turn it into 0% interest cash.

Should I include a contingency that says current tenants must sign a lease with me at or before closing, or is this going to kill the offer? 

I loves in a few during my undergrad years, it was nothing to have 250+ people in a half double 2 or 3 times per week for a little party, that’s just the norm in Stroudsburg.Parents sign the lease, renters insurance required, monthly checking, 1st, last and security.

This of course may not be in every case but if you not experience it will most probably turn out not really being in your own interest to attempt to do a lot of the work yourself or by using cheap unskilled labor. 

In may of this year I was going to increase the rent but I gave him a chose to pay $50 more per month or if he signed a two year contract I would keep the rent at $850/month.